feat: require approval at current PR head for merge (#471)

Add merge_approval_gate assessment so gitea_merge_pr fails closed when
visible APPROVED reviews do not pin the live head SHA. Expose
approval_at_current_head and stale_approval_block_reason in review
feedback. Document re-review requirement in canonical merge workflow.

Closes #471.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

skills/llm-project-workflow/workflows/review-merge-pr.md

@@ -762,6 +762,7 @@ Before merge, rerun fresh live checks:
 * author safety
 * PR re-fetch
 * reviewed head SHA unchanged
+* visible APPROVED review applies to the **current live PR head SHA** (`approval_at_current_head`); if the head moved after approval, re-review at the new head before merge (#471)
 * target branch freshly fetched
 * PR still open
 * PR still mergeable
@@ -778,6 +779,7 @@ Do not merge if:
 * capability state is stale
 * worktree is dirty
 * PR head changed
+* approval is stale (approved SHA ≠ current live head SHA)
 * validation failed
 * inventory was incomplete
 * PR is already landed