refactor(extension): 모듈 분리 + Hub 통합 테스트 #task-395

- extension.ts 3,446→1,289줄 (-63%)
- step-probe.ts (1,435줄): setupMonitor, processResponseFile, tryApprovalStrategies
- observer-script.ts (687줄): DOM observer script
- ws-client.ts (390줄): WSBridgeClient
- step-utils.ts (114줄): step 파싱 유틸
- auth.py (115줄): JWT + registration code
- hub.py (581줄): WSHub + per-client queue
- Hub WS 연동 테스트 통과 (auth, chat, register)
- VSIX v0.4.0 빌드

config.py

@@ -48,6 +48,10 @@ class Config:
     REMOTE_BRIDGE_URL: str = os.getenv("REMOTE_BRIDGE_URL", "")
     GATEWAY_API_KEY: str = os.getenv("GATEWAY_API_KEY", "")
 
+    # WebSocket Hub
+    GRAVITY_HUB_SECRET: str = os.getenv("GRAVITY_HUB_SECRET", "")  # JWT signing secret
+    GRAVITY_REGISTRATION_CODE: str = os.getenv("GRAVITY_REGISTRATION_CODE", "")  # Extension auth
+
     @classmethod
     def validate(cls) -> list[str]:
         """Return list of configuration errors."""