test: pin CONTRIBUTING.md's just build section to the recipe and the PR gates

[hivecommons-hive]

Test Improvement

Adds tests/contributing-just-build-parity.test.mjs, pinning the just build section of CONTRIBUTING.md to the two things it describes and never references.

CONTRIBUTING.md makes two claims nothing in the repository reads:

1. it enumerates, by name, the thirteen validate:* scripts the Justfile build recipe runs;
2. it states that recipe is not the "Validate repository" and "Lint repository" PR gates, and names the five commands a contributor must run as well — test:unit:coverage:check, check:format, check:spelling, check:markdown, check:audit.

tests/dev-environment.test.mjs pins the recipe to the workflows, but only through a validate:-prefixed filter and only against the Justfile; tests/contributing-e2e-setup.test.mjs pins the e2e setup instructions, not this section. So a validator added to the Justfile, or a check added to either gating job, leaves this section describing the repository as it used to be — and claim 2 is the only place the repository tells a contributor how to get a green check on the first push.

This is the drift-by-omission class already closed three times here: #1193 for the coverage thresholds (after #1037 and #1169/#1189), tests/audit-gate.test.mjs for the SECURITY.md allowlist, and tests/dev-environment.test.mjs for the Node major.

Design notes

• The validator list is set equality, not one-way. The siblings' rule — "documenting more than CI runs is never the bug" — fits a doc stating a minimum. This bullet is an enumeration of a recipe body, so a script named here that the recipe does not run is as wrong as one it runs and omits. The gate list stays one-way, matching tests/contributing-e2e-setup.test.mjs.
• Both searches are scoped to the bullet and the qualifying paragraph rather than the whole file. Several of these names also appear elsewhere in CONTRIBUTING.md, so a file-wide search would pass on a name that survived in an unrelated section after this one dropped it. This was not hypothetical — a file-wide draft passed while test:unit:coverage:check was unnamed.
• check:links is correctly out of scope: check-links.yml is weekly/dispatch only and deliberately does not gate PRs (CI workflow gaps: flaky-run traces are never uploaded and check:links never runs (needs human: workflow edits) #1188). Build aliases (build / build:production) are excluded too, since the recipe ends in the build step under its own alias.

The guard is shown to go red

Every comparison is a pure function of the three documents, so the fixtures can prove it fails on the drift it claims to catch — the standard tests/e2e-coverage-gate.test.mjs sets ("a ratchet nobody has seen go red is indistinguishable from one that always passes"). Four fixtures cover the drift itself:

fixture	caught as
validator added to the recipe alone	undocumented: ['validate:newly-added']
validator removed from the recipe alone	phantom: ['validate:awards']
gating check dropped from the paragraph	test:unit:coverage:check (run by the "validate" job)
check added to a gating job and nowhere else	check:newly-added (run by the "lint" job)

The remaining fixtures drive the "update this test" paths, so a restructured document is reported rather than crashed on three frames down.

Verification

• 15 tests in the new file pass; full unit suite green — 2388 passed, 0 failed.
• npm run test:unit:coverage:check passes; npx prettier --check clean.
• The new file measures 100.00% lines / 100.00% regions, taking the all-files region ratio from 96.17% to 96.20% — it adds margin to the aggregate gate rather than spending it.
• Parity holds exactly at 92cede9, so CONTRIBUTING.md, the Justfile and every workflow are untouched. This PR adds one file and changes nothing else.

Related Issue

Closes #1320

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Filed by quality agent (hold-gated mode). Human review required.

— hive: agent=quality backend=copilot model=claude-opus-5 copilot=1.0.88 requested_by=@mrbobbytables

[hivecommons-hive]

Important

Held for human review by the hive's ACMM level gate.

This PR was opened by the "quality" agent while Hive policy required a human checkpoint for that agent. Non-outreach agents are held at ACMM L3–L5; the outreach agent is always held because it publishes project-facing communication.

Hive will keep the hold label until a human removes it. Operators can make a deliberate one-off release during an ACMM level change with release_level_holds=true, but level changes never release this hold automatically.

[hivecommons-hive]

Supersession sweep found that this PR's claimed issue is already closed by another merged PR.

• Claimed issue: endusers#1193
• Closing PR: #1198

this PR also claims another issue that is still open. A human should decide whether to close this PR.

This comment is edited in place by the supersession sweep; it is not duplicated.