feat(lease): guarded cleanup for obsolete reviewer comment leases (Closes #691)

Add fail-closed assessment and MCP tool for expired/superseded-head
comment-backed reviewer leases that lack a control-plane lease_id.

- assess_obsolete_reviewer_comment_lease_cleanup eligibility matrix
- diagnose classifications: foreign_active_current_head,
  foreign_expired_current_head, foreign_completed_superseded_head,
  foreign_expired_superseded_head, orphaned_owner_missing,
  ambiguous_conflicting_evidence
- gitea_cleanup_obsolete_reviewer_comment_lease with confirmation
  CLEANUP OBSOLETE REVIEWER LEASE <n> and controller_recovery_authorized
- preserve audit history via append-only phase=released marker
- never repoint, transfer validation, use PID ownership, or steal
  active current-head foreign leases
- regression for PR #688 lease comment 10749 after recorded expiry

mcp_native_cleanup_proof.py

@@ -13,6 +13,7 @@ from typing import Any
 
 AUTHORIZED_CLEANUP_TOOLS = frozenset({
     "gitea_cleanup_post_merge_moot_lease",
+    "gitea_cleanup_obsolete_reviewer_comment_lease",
     "gitea_reconcile_merged_cleanups",
     "gitea_delete_branch",
     "gitea_cleanup_merged_pr_branch",