enhance(#3942): the emitted-drift ack is PR-lifetime data — move it to a commit trailer (#3954)

* test(#3942): failing-first suite for the emitted-drift ack commit trailer

Binds 37 input classes from the phase test matrix to the behavior ADR-3942
specifies, before any of it exists. Stubs return benign empty values rather
than throwing, deliberately: several rows assert that something DOES throw
(cap overflow, uncomputable commit range), and a throwing stub would turn
those green for the wrong reason and destroy the red.

The two rows that carry the design's load:

- merge-base semantics. The range is $(git merge-base base HEAD)..HEAD, not
  base..HEAD, because changedPaths comes from `git diff base...HEAD` (three
  dot). Two-dot would let the ack set and the change set disagree about which
  commits are this PR's. The fixture forks a topic branch, puts a trailer on
  each side, and asserts only the topic-side trailer is in range.

- fail-closed on an uncomputable range. With fragments a depth-1 checkout
  passes VACUOUSLY, every fragment reading as brand-new. With trailers the
  range cannot be computed at all, and returning an empty set would silently
  disarm the gate, so it must throw. The fixture builds a genuine shallow
  clone rather than simulating one.

Also covers the self-inflicted case: this change's own documentation quotes
the trailer syntax, so an example landing at the end of a commit message would
arm a live acknowledgment keyed on the literal placeholder text. Keys carrying
angle brackets or whitespace are rejected.

Authored per the phase artifacts 40-design.md and 50-test-matrix.md.
Not yet run on the remote runner — this commit exists to be tested.

Refs #3942

* chore(#3942): move the emitted-drift ack to a commit trailer

Implements ADR-3942, superseding ADR-2719 section 3 and its #2789 amendment.
Sections 1, 2 and 4-7 are retained: the conservation law is unchanged, only the
storage of its escape hatch moved off the working tree.

An acknowledgment explains one PR's ripple, and the moment that PR merges the
ripple is in the base, so it can never clear anything again. It was stored in
permanent shared state anyway, and every consequence of that mismatch had to be
built and then maintained. The chain is #2789 -> #2914 -> #3078 -> #3842 ->
#3823 -> #3875, each fix generating the next defect, ending in a scheduled
sweeper whose own first PR could not merge itself.

Added
  parseAckTrailers + renderAckTrailer (pure) and readAckTrailers (IO shell),
  reading Emitted-Drift-Ack-Hash: / Emitted-Drift-Ack-Growth: trailers over
  the merge-base range. tests/emitted-ack-trailer.test.cjs, 37 cases, written
  failing-first and confirmed red before any of this existed.

Changed
  diffEmitted takes two structurally distinct key-space maps instead of one
  shared paths map. That closes a latent defect: the spaces were separated by
  convention only, so a growth key satisfied a hash lookup by naming
  coincidence. staleAcks now reports which space a key was declared in.
  REMEDIATION teaches the trailer, per space, with its example rendered through
  renderAckTrailer so the taught grammar cannot drift from what the parser
  accepts.

Removed
  the sweep workflow, the guard-no-ack-on-next job, the standalone linter and
  its lint:ci entry, the fragment directory and its three spent fragments, the
  legacy single-file union, and the baseAck/spentAcks mechanism -- spentness is
  now structural, not computed.

Two range properties carry the design and are pinned by tests rather than
asserted: the range is merge-base scoped, matching git diff base...HEAD, so an
already-merged trailer is out of range by construction; and an uncomputable
range throws instead of reading as zero acknowledgments, which is the inverse
of the fragment guard's vacuous pass.

Three deliberate observable changes, each disclosed in the changeset: the
unread runtime field is gone, the legacy file is no longer read, and cross-space
excusal no longer works.

Ten open PRs carry fragments and will meet a modify/delete conflict. Measured
before landing and accepted deliberately; the one-line migration is in the PR
body.

Verified: lint:ci exit 0. Remote runner to follow on this exact sha.

Refs #3942

* fix(#3942): silent trailer collapse, lost coverage, and an unbounded cap

Six findings from the orthogonal review round, all fixed in place.

BLOCKER -- two trailers of the same name on one commit collapsed silently.
readAckTrailers built `separator=1d` where git needs `separator=%x1d`: the
`separator=` value inside a %(trailers:...) placeholder is itself a
pretty-format string, so the bare hex was emitted as two literal characters
and the split on \x1d never matched. Two same-name trailers therefore joined
into one value with errors empty -- the first reason absorbing the second
entry's key. Silent truncation, the exact class MAX_ACK_TRAILERS throws to
prevent. Confirmed with od -c against real git output before and after.

The failing-first matrix did not catch it because its "both spaces coexist"
row uses Hash plus Growth -- different trailer NAMES -- so the value separator
was never exercised. Two regression tests now cover same-name trailers
directly.

Coverage recovered: normalizeAckReason and INVISIBLE stayed on the live path
via parseAckTrailers but lost every test when the old suite was pruned. Back
under test against the current surface -- all six invisible codepoints
individually, whitespace collapse, trim, CRLF, and two seeded fast-check
properties. Dropping any single codepoint now fails.

MAX_ACK_TRAILERS counted raw trailers before de-duplication, so one trailer
carried forward across rebased commits counted once per commit and could throw
on a legitimate branch. Now counts distinct entries; 100 identical repeats
dedupe to one.

diffEmitted validated baseline, current and changedPaths but not the new
ackHash/ackGrowth, so a bad shape raised an unhandled TypeError instead of an
error verdict -- the same defect shape this file documents for #2778.

Docs: CONTRIBUTING and TESTING-SUITES were rewritten only in their first
sections; the later passages still taught fragments, git rm and the deleted
guard, contradicting the new text directly above them. Finished.

Also extends lint-removed-but-needed to exempt docs/adr and docs/research.
That gate fails on any docs mention of a file deleted in the same diff, which
makes it impossible to document a deletion in the PR performing it -- an ADR's
whole job is naming what it retired. Exemption is narrow and comes with a test
proving the gate still fires for a live consumer elsewhere under docs/. A
guard that cannot fail is worse than no guard. Maintainer-approved.

CONTEXT.md names the retired machinery by role rather than by filename: its
generated projection lands in docs/, which that gate does scan.

Adds docs/how-to/acknowledge-emitted-drift.md. The required docs set is
Reference and Explanation, so the task quadrant can be empty with every gate
green -- and this change has a real multi-step journey, including the fragment
migration ten open PRs now need.

lint:ci exit 0.

Refs #3942

* docs(#3942): correct the duplicate-trailer rule in CONTRIBUTING

Both axes of the code review independently flagged the same passage, without
seeing each other's output.

It claimed two declarations of the same key are always "a hard, loudly-reported
error, not a silent last-wins". That is only half true, and the missing half is
the one contributors hit: identical declarations -- same key, same reason --
dedupe silently, because a trailer legitimately survives a rebase and reappears
on every rebased commit. Failing there would red a branch for doing nothing
wrong, which is exactly why the dedup exists.

Only a same-key/different-reason pair errors, and that one is a genuine
ambiguity about which explanation holds.

As written, the paragraph told a contributor that a rebase-carried trailer
breaks the gate -- the opposite of the behavior. CONTEXT.md's parallel entry
already stated it correctly; this brings CONTRIBUTING into line.

Doc-only, root-level markdown.

Refs #3942

* chore(#3942): backfill changeset PR number to 3954

---------

Co-authored-by: sim <sim@local>

docs/TESTING-SUITES.md

@@ -137,32 +137,30 @@ The differential attribution check reports the file and the byte delta. To resol
 1. **Justify the growth in your PR** (a sentence in the description is enough) —
    the acknowledgment entry (below) is the review record that the larger size
    was a deliberate, seen decision, not silent drift.
-2. **Add an acknowledgment fragment** under `tests/emitted-drift-acks/` naming
-   the file and the reason, per `CONTRIBUTING.md`'s "Editing shipped content"
-   section and `CONTEXT.md`'s `### Emitted Artifact Provenance` entry. Name the
-   fragment for your issue or PR (something nobody else is using) — the failure
-   output prints a minimal valid document you can paste. This is deliberately a
-   per-PR fragment, not one shared file: two fragments can never *merge-conflict*
-   with each other, and a fragment appearing in your diff *is* the visible signal.
-   They do, however, share a path key space. If the failure instead names a path a
-   merged PR already acknowledged (a **spent** entry sitting in an existing
-   fragment), you have two routes and the error text names both: `git rm` that
-   fragment if every entry in it is spent — it gates nothing and only holds the
-   keys — or, if it is still live, reword/extend its `reason` in place to explain
-   the new ripple. Either way, do not add a duplicate entry for the same path; two
-   ack sources naming the same path is a hard, loudly-reported error.
-   The legacy single `tests/emitted-drift-ack.json` is still read and unioned
-   in for branches that carry it, but new acknowledgments never go there.
-3. **Your fragment is deleted once it has merged (#3078).** A fragment on `next`
-   is spent by definition — its prose is already at the base, so it can no longer
-   clear anything — while still owning its path keys, which walls off the next PR
-   that grows one of them. The `guard-no-ack-on-next` job reds `next` and prints
-   the exact `git rm` for every fully-spent fragment. A *partially* spent fragment
-   is deliberately left alone. Since #3875 you do not have to run that `git rm`:
-   the `ack-fragment-sweep` workflow (`.github/workflows/ack-fragment-sweep.yml`)
-   asks the guard for its own sweep list every six hours and opens a PR deleting
-   exactly what it named, holding back any fragment an open PR still touches
-   (#3842).
+2. **Add an acknowledgment trailer** to one of your own commits (ADR-3942),
+   naming the file and the reason, per `CONTRIBUTING.md`'s "Editing shipped
+   content" section and `CONTEXT.md`'s `### Emitted Artifact Provenance` entry:
+
+   ```
+   Emitted-Drift-Ack-Growth: explore.md — new dispatch section, reasoning ships with the block
+   ```
+
+   Growth keys on the **bare filename** as it appears under `gsd-core/workflows/`
+   or `agents/`; an unattributable **hash** ripple uses
+   `Emitted-Drift-Ack-Hash:` and keys on the emitted path (which always contains
+   a `/`). The two are separate namespaces — a growth trailer will not excuse a
+   hash ripple, and the failure output says which one applies. The trailer is the
+   review record that the larger size was a deliberate, seen decision.
+
+   If you need to change an acknowledgment, amend the commit carrying it. That is
+   deliberate: the trailer cannot drift out of sync with the diff it explains,
+   because changing either changes the sha and re-runs the gate.
+3. **There is nothing to clean up afterwards.** The trailer is read from
+   `git log $(git merge-base <base> HEAD)..HEAD` — your commits and no others —
+   so once your PR merges it is out of range by construction. It never becomes
+   "spent", it owns no shared key space, it cannot conflict with anyone else's,
+   and no sweeper has to delete it. That is the whole reason ADR-3942 moved the
+   acknowledgment off the working tree.
 4. **Or shrink it instead of acknowledging.** Prefer extraction when the growth
    is incidental: for a workflow, move per-mode bodies to
    `workflows/<name>/modes/`, templates to `workflows/<name>/templates/`, and
@@ -182,8 +180,7 @@ help — that is the signal to extract, per step 3.
 |---|---|
 | `scripts/workflow-size.cjs` | Single source of truth — LF-normalized byte counter (`lfByteCount`) + generic `measureMdFiles(dir, predicate)` (backs both workflows and agents) + workflow enumeration (`listWorkflowStems`, `measureWorkflows`). Imported by both guards and by `tests/helpers/emitted-runtime.cjs`'s `currentSizes()` so they can never measure differently. |
 | `tests/emitted-attribution.test.cjs` + `tests/helpers/emitted-diff.cjs` | The differential attribution check and its size ratchet (ADR-2719). The sole mechanism for both emitted-content propagation AND per-file size growth as of #2724. |
-| `tests/emitted-drift-acks/` | Per-PR acknowledgment fragments (primary, #2914) for unattributable emitted-content ripples and for size growth. A fragment appearing in your diff *is* the alarm; absence is the healthy steady state. |
-| `tests/emitted-drift-ack.json` | Legacy single acknowledgment file, superseded by the per-PR fragments above. Still read and unioned in for branches that carry it; must never gain new entries and must never persist on `next` (enforced by `guard-no-ack-on-next` via `scripts/lint-emitted-drift-ack.cjs --guard-next`). |
+| `Emitted-Drift-Ack-Hash:` / `Emitted-Drift-Ack-Growth:` commit trailers (ADR-3942) | The acknowledgment mechanism for unattributable emitted-content ripples and for size growth. Read from `git log $(git merge-base <base> HEAD)..HEAD` — no committed file, nothing to sweep; a merged trailer is out of range by construction. |
 | `npm run regen:derived` | Runs every remaining generator in dependency order (build → registry → ADR index → capability matrix → inventory manifest → manifest versions → `tests/fixtures/install-tree/*.json`). |
 | `tests/workflow-size-budget.test.cjs` | The workflow tier hard-cap guards, plus the `discuss-phase` progressive-disclosure checks. |
 | `tests/agent-size-budget.test.cjs` | The agent tier hard-cap guards (the agent analog). |