chore(#3211): accept a non-closing issue reference for docs/test-only PRs (#3289)

* test(#3211): failing-first coverage for the issue-link follow-up exemption

Adds the regression suite before the policy module exists, so the RED state
is recorded against a real verdict rather than asserted. Covers the reported
gap (a fork test-only follow-up PR cannot satisfy the gate without an inert
closing keyword) and the file-list truncation vector that any diff-shape
exemption must fail closed on.

Refs #3211

* chore(#3211): accept a non-closing issue reference for docs/test-only PRs

The `Issue link required` gate modelled exactly one PR->issue relationship —
"this PR closes that issue" — and its sole exemption additionally required
same-repo identity (#1389), so a fork PR had no exemption path of any kind. A
test-only or docs-only follow-up therefore had to ship a knowingly-inert
`Closes #<already-closed-issue>` to get a green check.

The verdict now lives in scripts/require-issue-link-policy.cjs as a pure,
unit-tested function returning a typed reason. It additionally accepts a
non-closing reference (`Refs #N`, `Follow-up to #N`, ...) but only when every
changed file is under tests/, under docs/, or is a root-level *.md — the same
doc-only shape pre-pr-gate.sh:111 recognizes, minus CHANGELOG.md, which
changeset/lint.cjs classes as user-facing. Source-touching PRs still require a
closing keyword and a PR with no reference at all still hard-fails, so gate
strength is unchanged.

Both constraints the issue names as hard requirements are preserved: the
backmerge exemption keeps its same-repo conjunct, and the failing step's `if:`
stays step-level so the required check reports SUCCESS rather than a
branch-protection-blocking `skipped`.

Also closes a forgery vector found while building this. `gh pr view --json
files` returns at most 100 paths and does not paginate, while the payload's
`changed_files` reports the true total (verified live: PR #3202 returns 100 of
118). A >100-file PR could therefore present a falsely tests-only list. The new
shared helper scripts/lib/pr-changed-files.cjs fails closed when the list
cannot be confirmed complete, and the pre-existing tooling-paths carve-out in
scripts/pr-template-policy.cjs — which relaxed template enforcement on the same
untrustworthy list — now uses it too.

Closes #3211

* fix(#3211): treat the authoritative file count as authority at every list size

Both orthogonal review passes independently found the same blocker.
`fileListIsComplete` only compared the list length against the PR's true
`changed_files` count once the list reached the 100-entry page cap, so any
mechanism that shortened the list BELOW the cap went undetected:

  evaluateIssueLink({prBody:"Refs #1", sameRepo:false,
                     changedFiles:["CONTRIBUTING.md"], changedFilesTotal:3})
  -> {ok:true, reason:"ok_followup_reference"}

The concrete exploit was a $GITHUB_OUTPUT heredoc collision. Both this
workflow and pr-template-format.yml wrote the file list with a fixed
terminator (`GSD_EOF` / the even weaker `EOF`), and every path in that value
is attacker-controlled on a fork PR. A file named after the delimiter closes
the value early and drops every path after it, so a fork PR touching src/
could present a list of only its exempt-looking files and take the follow-up
exemption. That is exactly the #1389 property this change is required to
preserve.

Fixed in two independent layers:

  1. The total is now the authority at every size, not only at/above the cap.
     One rule catches truncation, delimiter collision, and a path containing
     a newline, without having to enumerate the mechanisms.
  2. Both workflows now use an unguessable random delimiter, per GitHub's
     documented guidance for untrusted multiline output.

Also from review: pr-template-format.yml never passed CHANGED_FILES_TOTAL, so
the parameter threaded through evaluatePrTemplate was always undefined in
production and would have permanently blocked the tooling carve-out for any
100+-file PR; its env is now wired. Root-doc exclusion is case-insensitive.
Dropped a no-op `tr '\n' '\n'`.

Refs #3211

* chore(#3211): regenerate install-tree fixtures for the new shared helper

scripts/lib/** ships in the install tree, so adding
scripts/lib/pr-changed-files.cjs drifts all 19 golden fixtures by exactly one
path each. Caught by tests/golden-install-tree.test.cjs (25 failures on the
remote runner), which is the drift detector doing its job — not a defect.

Placement is deliberate: every existing occupant of scripts/lib/ is a CI/dev
helper that already ships (alias-drift-families, allowlist-ratchet, cli-exit,
drift-scan), so a shared helper used by two policy scripts belongs there. The
two policy modules themselves live at the top level of scripts/ and do not
ship.

Regenerated with `npm run gen:install-tree`; the delta is one added path per
fixture and nothing else.

Refs #3211

* fix(#3211): keep the shared CI helper out of the shipped install tree

The remote runner reported 6 failures on the previous head. Two causes.

`scripts/lib/**` is enumerated in `bin/install.js` (GSD_SCRIPTS_LIB_FILES) and
ships to users, and the install suite asserts that enumeration is complete.
Putting the new shared helper there broke four install tests and drifted all 19
golden install-tree fixtures. The right answer is not to add it to the manifest
— it is CI-only tooling used by two scripts that do not ship, so it has no
business in a user's config directory. Moved to `scripts/pr-changed-files.cjs`;
top-level `scripts/` ships only what the installer names explicitly, so nothing
is enumerated and nothing ships. The fixture regeneration from the previous
commit is reverted: the install-tree fixtures are byte-identical to `next`
again, and `bin/` is untouched. That also keeps the diff free of any
user-facing path, so no changeset fragment is required.

The other failure was a stale test, not a regression. The workflow carve-out
suite asserted the backmerge exemption by grepping require-issue-link.yml for
`startsWith(github.head_ref, ...)` and `steps.check.outputs.found`. This change
moved the whole verdict — carve-out included — into the policy module and
renamed the step, so those assertions measured a location the logic no longer
occupies.

Rewritten to lock the property at its new home, and made stronger in the
process: the step-level placement is now verified by PARSING the YAML and
asserting the job carries no `if:` of its own (a job-level `if:` would make the
required check report `skipped` and block branch protection), and the #1389
anti-forgery conjunct is asserted BEHAVIORALLY against evaluateIssueLink for
both sameRepo branches rather than by matching text. The bootstrap fallback
grep is locked too, so the introducing-PR path cannot be silently dropped.

Refs #3211

* fix(#3211): correct the contributor guidance and pin it against the rule

The sticky comment the gate posts still described the qualifying diff shape as
"nothing outside tests/ and docs/". The predicate had since been widened to
also accept root-level *.md, so the guidance was narrower than the rule it
describes — and narrower in the worst direction: a contributor whose PR is
CONTRIBUTING.md plus a test, which is exactly the shape #3211 was filed about,
would have been told they do not qualify while the gate was in fact passing
them. The two failure explanations now name all three accepted shapes and the
CHANGELOG.md exclusion.

This is a shared-rule-across-parallel-surfaces drift: the guidance restates a
rule whose definition lives in EXEMPT_PATH_PREFIXES / isRootLevelDoc /
EXCLUDED_ROOT_DOCS. It was caught by eye, which is not a control. Added the
parity assertion CLAUDE.md prescribes for exactly this: the test parses the
workflow, pulls the github-script body out of the failing step, and asserts it
names every entry of EXEMPT_PATH_PREFIXES and every entry of
EXCLUDED_ROOT_DOCS — derived from the module's exports, never from a second
hardcoded copy — plus the root-level shape and an actionable `Refs #` example.

The test is non-vacuous by construction and by demonstration: it guards against
zero-length iteration and an empty script body, and removing any single
expected token from the real text makes it fail (verified per token, plus the
empty-string case which reports all five missing).

Refs #3211

---------

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

CONTRIBUTING.md

@@ -189,6 +189,9 @@ Contributor requirements (summary):
 - **No draft PRs** — draft PRs are automatically closed. Only open a PR when it is complete, tested, and ready for review. If your work is not finished, keep it on your local branch until it is.
 - **Use the correct PR template** — there are separate templates for [Fix](.github/PULL_REQUEST_TEMPLATE/fix.md), [Enhancement](.github/PULL_REQUEST_TEMPLATE/enhancement.md), and [Feature](.github/PULL_REQUEST_TEMPLATE/feature.md). Using the wrong template or using the default template for a feature is a rejection reason.
 - **Link with a closing keyword** — use `Closes #123`, `Fixes #123`, or `Resolves #123` in the PR body. The CI check will fail and the PR will be auto-closed if no valid issue reference is found.
+  - **Test-only and docs-only follow-up PRs may reference without closing.** If your PR is documentation or regression coverage only — say, a repo-wide guard for a fix that already shipped — and there is no open issue for it to close, use a non-closing reference instead: `Refs #123`. `Ref`, `Refs`, `References`, `Relates to`, `Related to`, and `Follow-up to` are all accepted in that position. Do **not** write a closing keyword against an already-closed issue to satisfy the check; on merge it closes nothing, and it trains readers to treat closing keywords as decorative.
+  - **Qualifying diff shape:** every changed file must be under `tests/`, under `docs/`, or a root-level `*.md` (`README.md`, `CONTRIBUTING.md`, …). This mirrors the doc-only classification the push gate already uses, and it is deliberately root-only — markdown under a subdirectory (`gsd-core/workflows/*.md`, `agents/*.md`, `commands/**/*.md`) is runtime-loaded text, not documentation, so it still requires a closing keyword. `CHANGELOG.md` is excluded too: edit it through a `.changeset/` fragment, never directly.
+  - This weaker form is accepted **only** for that diff shape. A PR touching anything else still needs a closing keyword, and a PR with no issue reference at all still fails. On a very large PR (more than 100 changed files) the check cannot confirm the diff shape and falls back to requiring a closing keyword.
 - **One concern per PR** — bug fixes, enhancements, and features must be separate PRs
 - **No drive-by formatting** — don't reformat code unrelated to your change
 - **Don't bundle test-fixture updates into `docs:` or unrelated commits** — when a production change makes an existing test assertion stale, the test correction MUST land as its own `test:` (or `fix:`) commit, not bundled into a `docs:` commit that also updates the explanation. The release-sdk hotfix cherry-pick filter routes by commit-subject prefix (`fix:`, `chore:`, `test:`); a test-fixture correction packed under a `docs:` prefix is invisible to the picker and ships a half-state to the hotfix branch — production code changed, test assertion stale. v1.42.3 hit this exact mode (#3621). The fix is upstream: keep the test-fixture commit separate.