refactor(openclaw): remove REST facade (port 8004), unify on WebSocket

Remove the redundant OpenClaw REST service (port 8004) since frontend
already uses WebSocket via Gateway (port 8765) → OpenClaw (port 18789).

Deleted:
- backend/apps/openclaw_service.py
- backend/api/openclaw.py
- backend/tests/test_openclaw_service_app.py
- backend/tests/test_service_clients.py
- shared/client/openclaw_client.py

Updated:
- backend/apps/__init__.py — remove openclaw_app exports
- backend/api/__init__.py — remove openclaw_router
- shared/client/__init__.py — remove OpenClawServiceClient
- backend/services/gateway_openclaw_handlers.py — update docstring
- start.sh — remove port 8004 service startup

Architecture:
- Before: Frontend → HTTP :8004 → subprocess openclaw CLI
- After: Frontend → WS :8765 → Gateway → WS :18789 → OpenClaw

Constraint: Frontend already uses WebSocket exclusively
Confidence: high
Scope-risk: low (frontend unchanged)

shared/client/__init__.py

@@ -3,7 +3,6 @@
 
 from shared.client.control_client import ControlPlaneClient
 from shared.client.news_client import NewsServiceClient
-from shared.client.openclaw_client import OpenClawServiceClient
 from shared.client.runtime_client import RuntimeServiceClient
 from shared.client.trading_client import TradingServiceClient
 
@@ -12,5 +11,4 @@ __all__ = [
     "RuntimeServiceClient",
     "TradingServiceClient",
     "NewsServiceClient",
-    "OpenClawServiceClient",
 ]