enhance(#3172): require a stated failing direction for every automated acceptance command (#3825)

* test(#3172): failing-first suite for the stated failing-direction probe

Pins the <fails_when> pairing walk, placeholder denylist, MISSING sentinel
exemption, degraded-read contract, CLI arm and the plan-authoring contract text.
RED by construction: the module exports it requires do not exist yet.
Executed on the remote runner.

* feat(#3172): require a stated failing direction for every automated acceptance command

Every runnable <automated> command now carries a <fails_when> sibling naming
what output constitutes failure. A command with no expressible failure mode is
not an acceptance test: it reads as rigour and is not falsifiable.

- verify-command-grounding gains a failing-direction probe sharing the existing
  <automated> grammar, MISSING sentinel and walk guard rather than copying them
- gsd-tools check verify-failure-directions <N> backs it; plan-phase dispatches
  it and hands the JSON to gsd-plan-checker check 8f
- Dimension 8 detail extracted to references to stay under the agent size cap

Verified on the remote runner.

* fix(#3172): close four review findings in the failing-direction probe

- MISSING_SENTINEL_RE matched an env-var assignment prefix (MISSING=1 cmd), so
  a real command was exempted from the new blocking gate. Tightened the SHARED
  constant rather than adding a second copy.
- Both token regexes scanned to EOF on unclosed openers (O(n^2), 1562ms at 40k).
  Bodies are now non-crossing; 1ms, byte-identical on well-formed input. The
  pre-existing AUTOMATED_BLOCK_RE carried the same defect and is fixed here too.
- probePhaseFailingDirections reported status 'ok' when one plan was unreadable,
  conflating 'could not look' with 'nothing to report'.
- Extracted the phase-resolution block both check arms had copied verbatim.

Also corrects a docs/AGENTS.md dimension list stale since #2401.
Verified on the remote runner.

* fix(#3172): project the planner rule onto the spawn contract, settle emitted bookkeeping

The remote runner refuted the planner-side edit. agents/gsd-planner.md is frozen
under a 49152-LF-char cap asserted by four suites and sat at 49,146 — six chars
of headroom — so the +537 of authoring rule blew it. #3297/#3645 already settled
where such a rule goes: the planner spawn contract in plan-phase.md, beside
<tracked_source_paths>. The agent file is reverted to origin/next verbatim.

- plan-phase.md gains <failing_direction_contract>; tests row 30 now asserts the
  contract there and row 30b guards the freeze in both directions
- plan-phase.md growth acknowledged by APPENDING to the 3409 fragment, per the
  precedent that two ack sources may never name the same path
- install-tree fixtures regenerated for the three new reference files

Verified on the remote runner.

* chore(#3172): backfill PR number into the changeset fragment

pr:0 -> pr:3825 now that the PR exists.

---------

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

docs/COMMANDS.md

@@ -1269,6 +1269,55 @@ gsd-tools check verify-command-paths 3 --raw    # probe phase 3's verify command
 
 See [Resolve verify-command path findings](how-to/resolve-verify-command-path-findings.md).
 
+### `gsd-tools check verify-failure-directions`
+
+Deterministic presence probe over a phase's stated failing directions (#3172). Run automatically
+by `/gsd-plan-phase` before the plan-check pass and handed to `gsd-plan-checker`; runnable by
+hand to see what the checker saw.
+
+Every runnable `<automated>` command must carry a `<fails_when>` sibling naming what output
+constitutes failure. A command with no expressible failure mode is not an acceptance test.
+
+| Argument | Required | Description |
+|----------|----------|-------------|
+| `N` | **Yes** | Phase number whose `-PLAN.md` files are probed |
+
+| Flag | Description |
+|------|-------------|
+| `--raw` | Emit the JSON payload with no surrounding prose |
+
+**Prerequisites:** none — an unresolvable phase degrades to a JSON payload with `readError` set
+rather than failing.
+**Produces:** JSON on stdout. Nothing is written to disk.
+
+**It never executes command text**, and it never authors a statement for the planner — a
+prescribed failure signal would be copied verbatim and carry no information.
+
+**Pairing.** Within one `<task>`, each `<fails_when>` binds to the nearest **preceding**
+`<automated>`; the first statement after a command is the binding one. N runnable commands need
+N statements. A redundant second statement for the same command is ignored.
+
+Each row of `commands` carries `command`, `statement`, `plan`, `task`, `status`, and `severity`.
+
+| `status` | `severity` | Meaning |
+|---|---|---|
+| `ok` | `none` | A non-empty, non-placeholder statement is bound to this command |
+| `missing` | `blocker` | The command has no `<fails_when>` at all |
+| `empty` | `blocker` | A `<fails_when>` is present but blank |
+| `placeholder` | `blocker` | The whole statement is `TBD`, `TODO`, `N/A`, `NA`, `none`, `unknown`, `TBA`, `?`, or `-` (case-insensitive, whole value only) |
+| `orphan` | `warning` | A `<fails_when>` that follows no command — it satisfies nothing |
+| `sentinel` | `none` | A Nyquist `MISSING — Wave 0 …` placeholder; not runnable, so exempt |
+
+The top-level `status` is `blocked` when any row is a blocker, `unresolvable` when the probe
+could not look, and `ok` otherwise. A non-empty `readError` means the probe **could not look** —
+distinct from finding nothing.
+
+```bash
+gsd-tools check verify-failure-directions 3 --raw    # probe phase 3's failing directions
+```
+
+See [State a failing direction](how-to/state-a-failing-direction.md).
+
 ---
 
 ## Workstream Management