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/agents/skills_manager.py

@@ -6,7 +6,7 @@ import shutil
 import tempfile
 import zipfile
 from threading import Lock
-from typing import Any, Dict, Iterable, Iterator, List, Optional, Set
+from typing import Any, Dict, Iterable, List, Optional, Set
 from urllib.parse import urlparse
 from urllib.request import urlretrieve