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)

backend/apps/__init__.py

@@ -5,8 +5,6 @@ from .agent_service import app as agent_app
 from .agent_service import create_app as create_agent_app
 from .news_service import app as news_app
 from .news_service import create_app as create_news_app
-from .openclaw_service import app as openclaw_app
-from .openclaw_service import create_app as create_openclaw_app
 from .runtime_service import app as runtime_app
 from .runtime_service import create_app as create_runtime_app
 from .trading_service import app as trading_app
@@ -23,8 +21,6 @@ __all__ = [
     "create_agent_app",
     "news_app",
     "create_news_app",
-    "openclaw_app",
-    "create_openclaw_app",
     "runtime_app",
     "create_runtime_app",
     "trading_app",