Promote H21 (negation-window closing-keyword detection) from report-only patrol row to the blocking PR gate, once the patrol baseline is long enough

[huangyiirene]

Follow-up filed by the skills seat (session_01ApyDuQY2fkunMCqXiqvBhR) on ACCEPTing the H21 report-only row (PR #10939, card #10392) — the stage-2 promotion decision, tracked as a card per the landing seat's follow-up duty.

What

Wire h21NegatedClosingKeyword (exported from scripts/pm/check-half-states.mjs) into the blocking PR gate surface — scripts/check-partof-closing-keyword.mjs + .github/workflows/partof-closing-keyword-guard.yml — with its own self-test cases and its own remedy string. The H21 predicate itself needs no change; the work is the gate wiring, the gate's self-test, and the remedy text.

Why hold, and the exit

The stage-1 measurement behind the row is clean (0 false positives in 301 closing-keyword matches over the 300 most recently merged PR bodies; window sensitivity measured; per-marker independence measured — full numbers in PR #10939's header comment and the #10392 report). But that is a 2.2-day window of one repo's register, and a wrong blocking gate hard-reds unrelated PRs and teaches authors to route around it, while a wrong patrol row costs one report line. The promotion should ride a longer baseline, not a moment.

Restart-when: after 2026-09-04, the half-state patrol runs since H21 landed report 0 H21 false positives (a flagged PR whose sentence was in fact a correct close counts as one; the specimen class does not)

Constraints for the eventual dispatch

• The dispatch surface is the GATE side only; ⛔ do not restructure H21 or re-run its corpus inside check-half-states.mjs beyond what wiring requires.
• The remedy string must keep the safe rewordings and the backtick advice — backticks are valid on the PR-body surface this gate reads (unlike the commit-message surface, which is [finding] The closing-keyword guards read PR BODIES only — a commit message is a second surface GitHub closes cards from, and nothing in the repo checks it #10942's separate card).
• If any H21 false positive appears in the patrol before the restart date, this card's premise is re-triaged, not silently discharged.

Ref: #10392 (the commissioned two-stage order) · PR #10939 (the row and its measurements) · #10942 (the commit-message surface, deliberately not this card)