refactor(games): rework room lifecycle events and remove chess plugin

Consolidate room leave/delete event handling into RoomManager emitter,
remove redundant PLAYER_LEFT publishes from GameServer, and delete the
chess game plugin (board, types, tests) in favor of the new plugin
architecture. Add per-module CLAUDE.md files for leveling, guild-settings,
feature-flags, db, api, and panel to improve agent navigability.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

panel/src/lib/useGameRoom.ts

@@ -22,9 +22,11 @@ export function useGameRoom(roomId: string, userId: string, role?: string, prefe
     const { send, subscribe, connected } = useWebSocket();
     const navigate = useNavigate();
     const navigateRef = useRef(navigate);
+    const errorTimerRef = useRef<ReturnType<typeof setTimeout>>();
     useEffect(() => {
         navigateRef.current = navigate;
     }, [navigate]);
+    useEffect(() => () => clearTimeout(errorTimerRef.current), []);
 
     const [state, setState] = useState<GameRoomState>({
         gameState: null,
@@ -121,6 +123,10 @@ export function useGameRoom(roomId: string, userId: string, role?: string, prefe
                         setTimeout(() => navigateRef.current("/games"), 2000);
                     } else {
                         setState(prev => ({ ...prev, error: msg.message }));
+                        clearTimeout(errorTimerRef.current);
+                        errorTimerRef.current = setTimeout(() => {
+                            setState(prev => ({ ...prev, error: null }));
+                        }, 5000);
                     }
                     break;
             }
@@ -133,7 +139,11 @@ export function useGameRoom(roomId: string, userId: string, role?: string, prefe
     }, [roomId, connected, userId, send, subscribe]);
 
     const sendAction = useCallback((action: unknown) => {
-        send({ type: "GAME_ACTION", roomId, action });
+        const sent = send({ type: "GAME_ACTION", roomId, action });
+        if (!sent) {
+            setState(prev => ({ ...prev, error: "Not connected — action not sent." }));
+            return;
+        }
         setState(prev => ({ ...prev, error: null }));
     }, [roomId, send]);