refactor(cleanup): remove legacy agent classes and complete EvoAgent migration

Remove deprecated AnalystAgent, PMAgent, and RiskAgent classes.
All agent creation now goes through UnifiedAgentFactory creating EvoAgent instances.

- Delete backend/agents/analyst.py (169 lines)
- Delete backend/agents/portfolio_manager.py (420 lines)
- Delete backend/agents/risk_manager.py (139 lines)
- Update all imports to use EvoAgent exclusively
- Clean up unused imports across 25 files
- Update tests to work with simplified agent structure

Constraint: EvoAgent is now the single source of truth for all agent roles
Constraint: UnifiedAgentFactory handles runtime agent creation
Rejected: Keep legacy aliases | creates maintenance burden
Confidence: high
Scope-risk: moderate (affects agent instantiation paths)
Directive: All new agent features must be added to EvoAgent, not legacy classes
Not-tested: Kubernetes sandbox executor (marked with TODO)

backend/services/gateway_openclaw_handlers.py

@@ -24,7 +24,7 @@ import logging
 from typing import TYPE_CHECKING
 
 if TYPE_CHECKING:
-    from backend.services.gateway import Gateway
+    pass
 
 logger = logging.getLogger(__name__)
 
@@ -88,7 +88,6 @@ def _ensure_session_bridge(gateway) -> None:
 
 def _get_ws_client(gateway) -> "OpenClawWebSocketClient":
     """Get the OpenClaw WebSocket client from gateway."""
-    from shared.client.openclaw_websocket_client import OpenClawWebSocketClient
     client = gateway._openclaw_ws
     if client is None:
         raise RuntimeError("OpenClaw Gateway not connected")