fix: make drawer_id deterministic for idempotent writes

Remove datetime.now() from drawer_id hash so same content + wing + room
always produces the same ID. This enables the idempotency check that
returns "already_exists" on duplicate writes.

mempalace/mcp_server.py

@@ -330,7 +330,7 @@ def tool_add_drawer(
     if not col:
         return _no_palace()
 
-    drawer_id = f"drawer_{wing}_{room}_{hashlib.sha256((content[:100] + datetime.now().isoformat()).encode()).hexdigest()[:24]}"
+    drawer_id = f"drawer_{wing}_{room}_{hashlib.sha256((wing + room + content[:100]).encode()).hexdigest()[:24]}"
 
     _wal_log(
         "add_drawer",