fix(queue): re-fetch PR head before merge to detect stale SHA

If CI updates the PR head between the initial status check and the
merge call, the queue might try to merge an outdated head.  Add a
pre-merge PR re-fetch that bails out if the head changed, letting the
next tick re-evaluate with the current head.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitea/scripts/gitea-merge-queue.py

@@ -481,6 +481,13 @@ def process_once(*, dry_run: bool = False) -> int:
         )
         return 0
     if decision.ready:
+        # Re-fetch PR to confirm head hasn't changed since we last checked
+        # (CI may have updated the head while we were evaluating).
+        current_pr = get_pull(pr_number)
+        current_head = current_pr.get("head", {}).get("sha", "")
+        if current_head != head_sha:
+            print(f"::notice::PR #{pr_number} head changed {head_sha[:8]} -> {current_head[:8]}; re-evaluating")
+            return 0
         latest_main_sha = get_branch_head(WATCH_BRANCH)
         if latest_main_sha != main_sha:
             print(