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.py

@@ -13,9 +13,7 @@ from typing import Any, Callable, Dict, List, Optional, Set
 import websockets
 from websockets.asyncio.server import ServerConnection
 
-from backend.data.provider_utils import normalize_symbol
 from backend.domains import news as news_domain
-from backend.llm.models import get_agent_model_info
 from backend.core.pipeline import TradingPipeline
 from backend.core.state_sync import StateSync
 from backend.services.market import MarketService
@@ -146,7 +144,7 @@ class Gateway:
         self.state_sync.update_state("status", "websocket_ready")
 
         # Create server but don't block yet - we'll serve inside the context manager
-        server = await websockets.serve(
+        await websockets.serve(
             self.handle_client,
             host,
             port,