fix: wave 5b review blockers - suppressed memory key, finalizer merge, zero-image guard

backend/agents/memory.py

@@ -7,7 +7,8 @@ import threading
 from typing import Any, Dict, Iterable, Optional
 
 
-_COLLECTION_KEYS = {"sheets", "clusters", "findings", "decisions", "rfis"}
+_COLLECTION_KEYS = {"sheets", "clusters", "findings", "decisions", "rfis",
+                    "suppressed"}
 _MAPPING_KEYS = {"sheet_index", "jurisdiction", "object_graph"}
 _MEMORY_KEYS = _COLLECTION_KEYS | _MAPPING_KEYS