Re-verify the clean review comment on GitHub before MERGE writes

Follow-up from #68 / PR #93.

Context

Since #68 the merge gate requires the persisted binding (reviewed_pr_url, reviewed_head_sha, reviewed_base_ref) and checks current_pr_url and the PR GitHub returns against it by identity. The binding is written by _apply_review after the review comment was verified on GitHub (right PR, right round, right HEAD, unique marker), and MERGE trusts that persisted record.

Gap

The gate does not re-read the review comment at merge time. A state file whose last_review_result, reviewed_* fields and last_review_comment_url were edited by hand (or written by a controller bug) after the review is accepted by the gate as long as the fields are shape-valid and consistent with current_pr_url and the PR's current HEAD/base. GitHub is the source of truth for every other claim before MERGE; the clean review itself is the one fact still taken from state alone.

Proposed direction

Before READY_FOR_MERGE accepts and before MERGE writes, re-read last_review_comment_url through the GitHub client and require: the comment exists on the reviewed PR (same_target), its ai-review-result marker names review_round and reviewed_head_sha, and needs_fix_round is false. Mismatch → BLOCKED (conclusive), a transient read failure → the bounded inconclusive path already used by the gate. Reuse the marker parsing in claims.py / _verify_review_comment; dry-run stays read-only.

Acceptance

• A state parked in READY_FOR_MERGE whose review comment is missing, is on another PR, names another round/HEAD, or says needs_fix_round: true → BLOCKED, no merge_pr call.
• The happy path performs exactly one extra comment read per gate pass.
• docs/agent-guides/github-safety.md "Before MERGE" gains the bullet.

Start with claims.py and _verify_review_comment, then trace the READY_FOR_MERGE and MERGE gate paths described in the issue. Check the existing bounded inconclusive handling and dry-run behavior before adding the extra comment read. Done means the listed mismatches block without merge_pr, transient failures use the existing path, the happy path reads once, and docs/agent-guides/github-safety.md includes the new Before MERGE bullet.