Bind a PR's review PASS to its tip SHA (agent-merge gate) #215

Goal (one observable behavior)

A merged pull request can be checked, mechanically, against the review it claims: a Verdict: PASS report that names the current PR head SHA. Before: "reviewed tip" is a rule in AGENTS.md and a skill's honesty contract, and nothing verifies it at merge time. After: a read-only, report-only workflow prints, for every PR, whether the tip has an independent PASS, and says why not when it does not.

This is the mechanism that makes standing auto-merge delegation for T0/T1 safe: an agent may enable auto-merge only when the gate can show a PASS at the tip being merged.

Acceptance criteria

• .github/scripts/review-gate.mjs with pure, exported functions (parse a report, pick the deciding review, bind it to the head SHA), a --pr N by-hand mode and a --sha override, following risk-gate.mjs's shape.
• Unit tests (node --test .github/scripts/review-gate.test.mjs, already swept by npm run verify) that fail without the change: stale SHA, self-review by the PR author, FAIL after PASS, non-reviewer author, missing tip, missing Ran:.
• .github/workflows/review-gate.yml: pull_request, read-only token, base-branch code, base-missing fallback like risk-tier.yml, continue-on-error: true, not a required check and not in tests-ok's needs.
• docs/autonomous-dev.md describes the record; AGENTS.md step 6 states the standing T0/T1 delegation and that the gate is its evidence.

Where it sits (file:line pointers)

• .github/scripts/risk-gate.mjs (shape to follow), .github/workflows/risk-tier.yml (workflow shape)
• .claude/skills/review-pr/SKILL.md (the report format the gate parses — unchanged)
• AGENTS.md step 6, docs/autonomous-dev.md phases

Out of scope / do not touch

• No LLM in Actions; no secrets; no new credentials (that is the delegation slice).
• Do not make it a required check yet, and do not touch tests-ok, branch protection, or test.yml.
• No posting, approving, merging or labeling from this workflow.