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/api/runtime.py

@@ -7,7 +7,6 @@ import asyncio
 import json
 import logging
 import os
-import signal
 import shutil
 import subprocess
 import sys
@@ -20,7 +19,6 @@ logger = logging.getLogger(__name__)
 from fastapi import APIRouter, BackgroundTasks, HTTPException, Request
 from pydantic import BaseModel, Field
 
-from backend.runtime.agent_runtime import AgentRuntimeState
 from backend.config.bootstrap_config import (
     resolve_runtime_config,
     update_bootstrap_values_for_run,