docs(pm-dispatch): harden gate-label writes to read-modify-write + read-back

[os-zhuang]

Fixes#9251

ADR-class skills-root PR — awaiting a human merge. Per the skills-update ruling (2026-08-11) this PR must not be merged, queued, or armed for auto-merge by any AI seat; it stays draft and visibly parked for the maintainer.

Scope (narrowed by the 2026-08-18 maintainer ruling recorded on the issue)

Short-term hardening only. The gate carrier stays the needs:contract-review label; the card's Option 2 (carrier migration to PR review / Request Changes) is closed by the ruling — verbatim: 「中期把闸门迁到 PR review 的 Request Changes 上 我觉得没必要」 — and this PR does not implement it or leave a door open for it.

What changed — .claude/skills/pm-dispatch/SKILL.md, 2 edits, zero net lines (682 → 682, ratchet headroom 0)

1. State-model label discipline bullet (「写后回读按风险定向」): upgraded from "label writes always read back" to a mandatory hard four-step for every label write — fetch the current set, add/remove only the intended label, write the merged set, read back to verify — with the rationale spelled out: a whole-set PUT writes this seat's snapshot, so a concurrent seat's label landing between your read and your write is silently stripped, often a label you never meant to touch. Gate-carrying labels (needs:contract-review) are named explicitly, both directions (apply and clear): a stripped gate is not a red light but a silent release, and "stripped" vs "never applied" are evidentially indistinguishable — read-back is the only detection.
2. needs:contract-review review-chain bullet (入队与落地): appended the carrier ruling with date and verbatim quote — the gate carrier stays this label, no migration to PR review / Request Changes, no door left open — and bound both write directions (apply and clear) to the hard read-modify-write step above.

No changeset: .claude/ internal agent tooling only, nothing published (skip-changeset).

Verification (all at HEAD 39310dc21)

• pnpm check:pm-skill-ratchet — green, SKILL.md 682 lines (ceiling 682, headroom 0; both edits are in-place line replacements, no re-wrap used to fund lines)
• pnpm check:pm-skill-id-lint — green, 9 files clean
• pnpm check:doc-authoring — green, 377 files clean
• pnpm check:skill-frame-sync — green, 4 copies isomorphic
• pnpm --filter @objectstack/lint run check:doc-formula-expressions — green (self-test 24 cases + 22 examples clean; dependency closure built first)
• pnpm check:nul-bytes — green, 6130 files scanned
• node scripts/check-adr-merge-approval.mjs — will be red on this PR until an approving review exists; that is the gate's designed state for a .claude/skills/** diff, not a defect of this change
• Gate re-derivation on the actual diff (node scripts/pm/dispatch-gates.mjs .claude/skills/pm-dispatch/SKILL.md) matched the dispatched list exactly; no additional families

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