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]>

gitea_mcp_server.py

@@ -535,6 +535,7 @@ import issue_lock_worktree  # noqa: E402
 import issue_lock_provenance  # noqa: E402
 import issue_lock_store  # noqa: E402
 import issue_lock_adoption  # noqa: E402
+import merge_approval_gate  # noqa: E402
 import already_landed_reconcile  # noqa: E402
 import author_mutation_worktree  # noqa: E402
 import issue_claim_heartbeat  # noqa: E402
@@ -2472,6 +2473,10 @@ def gitea_get_pr_review_feedback(
         e for e in latest_by_reviewer.values()
         if e["verdict"] == "APPROVED" and not e["dismissed"]
     ]
+    approval_head = merge_approval_gate.assess_merge_approval_head(
+        current_head_sha=current_head,
+        latest_by_reviewer=latest_by_reviewer,
+    )
     return {
         "success": True,
         "pr_number": pr_number,
@@ -2482,6 +2487,9 @@ def gitea_get_pr_review_feedback(
             login: e["verdict"] for login, e in latest_by_reviewer.items()},
         "has_blocking_change_requests": bool(blocking),
         "approval_visible": bool(approvals),
+        "approval_at_current_head": approval_head["approval_at_current_head"],
+        "latest_approved_head_sha": approval_head["latest_approved_head_sha"],
+        "stale_approval_block_reason": approval_head["stale_approval_block_reason"],
         "latest_reviewed_head_sha": latest_reviewed_head,
         "review_feedback_stale": bool(
             latest_reviewed_head and current_head
@@ -3442,6 +3450,9 @@ def gitea_merge_pr(
             result["permission_report"] = feedback["permission_report"]
         return result
     result["approval_visible"] = feedback.get("approval_visible")
+    result["approval_at_current_head"] = feedback.get("approval_at_current_head")
+    result["latest_approved_head_sha"] = feedback.get("latest_approved_head_sha")
+    result["review_feedback_stale"] = feedback.get("review_feedback_stale")
     result["has_blocking_change_requests"] = feedback.get(
         "has_blocking_change_requests")
     if feedback.get("has_blocking_change_requests"):
@@ -3455,6 +3466,16 @@ def gitea_merge_pr(
             "completed before merge (fail closed)"
         )
         return result
+    if not feedback.get("approval_at_current_head"):
+        reasons.append(
+            feedback.get("stale_approval_block_reason")
+            or (
+                "approval does not apply to current PR head SHA "
+                "(fail closed); required next action: re-review PR at "
+                "current head before merge"
+            )
+        )
+        return result
 
     # Gate 8 — in-process mutation authority (#199): the last check before
     # the merge mutation, using the identity the eligibility gate proved.