fix: preserve default KG path when --palace not passed

When --palace is not explicitly provided, fall back to KnowledgeGraph()
which uses DEFAULT_KG_PATH (~/.mempalace/knowledge_graph.sqlite3),
preserving backward compatibility for existing users.

mempalace/mcp_server.py

@@ -53,7 +53,10 @@ if _args.palace:
     os.environ["MEMPALACE_PALACE_PATH"] = os.path.abspath(_args.palace)
 
 _config = MempalaceConfig()
-_kg = KnowledgeGraph(db_path=os.path.join(_config.palace_path, "knowledge_graph.sqlite3"))
+if _args.palace:
+    _kg = KnowledgeGraph(db_path=os.path.join(_config.palace_path, "knowledge_graph.sqlite3"))
+else:
+    _kg = KnowledgeGraph()
 
 
 def _get_collection(create=False):