Label merged pull requests with pass when the verdict was PASS #739

Problem

A merged pull request carries no lasting trace of the verdict that let it in: agent-pr stays, the verdict lives only in a comment thread, and once the branch is gone the review record is hard to read from the pull request list. We want every pull request that merged on a PASS verdict to be visibly marked, and the marking to be mechanical rather than typed by hand.

What to build

1. Add a pass label to .github/labels.json (description along the lines of Reviewer PASS verdict; merged through the merge queue, a colour distinct from risk:*), so sync-labels.mjs creates it.
2. Apply it automatically when the verdict is posted: the code path that already parses Verdict: (review-gate.mjs, or the workflow that runs it) adds pass to the pull request when the comment's first line is Verdict: PASS at <sha>.
   • a Correction from … comment must not add the label.
   • a Replaces: PASS adds it like a normal PASS.
   • a later INC on a newer tip does not remove it (the label records that a PASS was given; hold is still the only manual stop).
3. Backfill: --backfill mode (or a one-shot script) that labels already-merged pull requests whose comment history contains a PASS verdict at their merged head sha, so the existing record is visible without hand work.

Acceptance

• posting Verdict: PASS at <sha> on a test pull request results in the pass label appearing, with no other label changed.
• Written-by: / Ran: lines and the third-line Posted-by: attribution do not affect it.
• backfill run against this repository labels the pull requests that merged on a PASS and leaves INC-only ones alone; a second run adds nothing.
• unit test: a PASS comment labels, an INC comment does not, a Correction from comment does not.

Out of scope

Retro-active inc label, changes to what makes a PASS, and the merge queue itself.