fix(merge gate): bind pr_merge and tbd_ship merges to the head commit the gate checked (TOCTOU)

The gate evaluates CI for one head sha; a push to the PR branch between that check and the merge would have merged
unchecked code. MergePRArgs now carries head_commit_id (documented in Gitea 1.27.1's MergePullRequestOption) and both
tools send the sha they evaluated. Tests for both tools; each removal flip-tested red.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SZkhi49hfeSqBp5aZJNXsM

internal/tools/tbd_ship.go

@@ -320,7 +320,7 @@ func (t *TBDShip) Call(ctx context.Context, raw json.RawMessage) (json.RawMessag
 	}
 
 	// Green + unprotected → squash-merge, then delete the short-lived branch.
-	if err := t.c.MergePullRequest(ctx, args.Owner, args.Repo, pr.Number, gitea.MergePRArgs{Do: "squash"}); err != nil {
+	if err := t.c.MergePullRequest(ctx, args.Owner, args.Repo, pr.Number, gitea.MergePRArgs{Do: "squash", HeadCommitID: pr.Head.Sha}); err != nil {
 		if errors.Is(err, gitea.ErrConflict) {
 			result["reason"] = "base moved and the merge is not clean — PR left open, not merged; resolve the conflict and merge manually"
 			return textOK(result)