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/api/__init__.py

@@ -11,7 +11,6 @@ Provides REST API endpoints for:
 from .agents import router as agents_router
 from .workspaces import router as workspaces_router
 from .guard import router as guard_router
-from .openclaw import router as openclaw_router
 from .runtime import router as runtime_router
 from .runs import router as runs_router
 
@@ -19,7 +18,6 @@ __all__ = [
     "agents_router",
     "workspaces_router",
     "guard_router",
-    "openclaw_router",
     "runtime_router",
     "runs_router",
 ]