feat(pm): add H18 — pm:retriage aged past the triage cycle

[qq9340100]

Fixes#10082

What

Adds H18 to scripts/pm/check-half-states.mjs: a report-only row for an
open card carrying pm:retriage past a 2h threshold (RETRIAGE_STALE_HOURS,
aligned to the triage Routine's hourly re-judgement cadence — same reasoning
as DOMAIN_HALF_STATE_STALE_HOURS/H13). Maintainer ruling 2026-08-19/20,
verbatim: 「同意 并存」— pm:retriage coexists with the card's standing pm:*
label rather than replacing it, and the triage Routine is the sole remover.
Without an age check that label becomes the next half-state nobody watches.

• The row names the card and its coexisting standing pm:* label(s).
• pm:retriage present with no coexisting pm:* label gets its own note
  instead — the coexistence the state model requires is itself missing, so
  the disputed grading is unidentifiable (异议对象不明).
• Age is read from updated_at (the same timeline-fetch-free proxy H13
  uses) — every triage re-judgement bumps it, so a stale reading means
  nothing touched the card since any triage pass.
• Wired into the existing unscoped pass (H13-H15) — pm:retriage can
  coexist with a label this sweep's label pages never fetch (e.g.
  pm:blocking) or with none at all, so only the unscoped listing is
  guaranteed to see every carrier. No new fetch.
• Report-only, ordinary row in both renderers (plain + markdown), never loud
  — same property H14-H16 hold.

Tests

pnpm check:pm-half-states (== node scripts/pm/check-half-states.mjs --self-test) at e47badf: 489 cases pass (469 baseline before this PR,
re-measured on origin/main at dispatch time, matching the #10082 handoff
comment's 414→469 note; +20 new H18 cases here).

New cases cover: over-threshold hit (single and multiple coexisting pm:*
labels), under-threshold no-hit, exactly-at-threshold (clean), no
pm:retriage label (out of scope), the disputed-target variant
(pm:retriage alone — asserts the 异议对象不明 note and that no coexisting
label is falsely claimed), a domain:* label not counted as a coexisting
pm:* label, unreadable/absent updated_at (#4690: must not read as
fresh), not-loud, and the plain/markdown ordinary-row rendering shapes
(mirroring the H14/H15/H16 pins).

✓ check-half-states self-test: 489 cases pass.

Also green at the same head:

• node scripts/check-cross-package-test-inputs.mjs — OK: 12 package(s) read outside themselves, all declared, and turbo.json hashes every declared glob.
• pnpm check:nul-bytes — OK (scanned 6066 text file(s) ... no raw ASCII control bytes)

node scripts/check-partof-closing-keyword.mjs requires PR_BODY/PR_NUMBER
(a PR-body linter with no local dry-run target) — validated locally against
this exact body via PR_BODY="$(cat pr-body.md)" node scripts/check-partof-closing-keyword.mjs: ✓ this PR carries no Part-of/ closing-keyword contradiction. It also runs in CI on
.github/workflows/partof-closing-keyword-guard.yml.

Scope

Non-governed surface (internal tooling): skip-changeset.

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