feat(coord): project verdicts to GitHub checks and gate merge consumption

Parent: #384 (P0-3b). Depends on P0-3a.

## Scope
Project authoritative gateway verdicts to a GitHub check and make coord-main's merge gate consume that check for the current head SHA.

## Acceptance
- Required check is green only for complete non-conflicting current-SHA verdicts.
- New commits invalidate the previous check.
- GitHub labels/comments cannot substitute for the gateway verdict.
- Projection retry is idempotent and failures remain visible.

Delivery discipline: one issue, one worker branch, one PR.

Start by reading parent issue #384 and the dependency P0-3a, then locate coord-main's merge gate and the GitHub check projection entry point. Verify the acceptance cases for current-SHA verdicts, invalidation after new commits, label/comment rejection, idempotent retries, and visible failures.

Written by the indexing model from the issue text.