feat: require post-merge cleanup safety-gate proof in reviewer reports (Closes #402)

Adds post_merge_cleanup_proof validation so cleanup claims must carry the
branch deletion and worktree removal checklists, or report CLEANUP_SKIPPED
with an exact blocker.

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

final_report_validator.py

@@ -11,6 +11,7 @@ import inspect
 import re
 from typing import Any, Callable
 
+from post_merge_cleanup_proof import assess_post_merge_cleanup_proof
 from review_proofs import (
     HANDOFF_HEADING,
     assess_controller_handoff,
@@ -889,6 +890,20 @@ def _rule_reviewer_review_mutation(
     )
 
 
+def _rule_reviewer_post_merge_cleanup_proof(report_text: str) -> list[dict[str, str]]:
+    result = assess_post_merge_cleanup_proof(report_text)
+    if not result.get("block"):
+        return []
+    return _findings_from_reasons(
+        "reviewer.post_merge_cleanup_proof",
+        result.get("reasons") or [],
+        field="Cleanup status",
+        severity="block",
+        safe_next_action=result.get("safe_next_action")
+        or "report CLEANUP_SKIPPED with blocker or full cleanup checklist",
+    )
+
+
 _RULES_BY_TASK: dict[str, list[Callable[..., list[dict[str, str]]]]] = {
     "review_pr": [
         _rule_shared_controller_handoff,
@@ -909,6 +924,7 @@ _RULES_BY_TASK: dict[str, list[Callable[..., list[dict[str, str]]]]] = {
         _rule_reviewer_target_branch_freshness,
         _rule_reviewer_mutation_ledger,
         _rule_reviewer_review_mutation,
+        _rule_reviewer_post_merge_cleanup_proof,
     ],
     "reconcile_already_landed": [
         _rule_reconcile_controller_handoff,