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/base/command_handler.py

@@ -8,7 +8,7 @@ import logging
 from abc import ABC, abstractmethod
 from dataclasses import dataclass, field
 from pathlib import Path
-from typing import TYPE_CHECKING, Any, Callable, Dict, List, Optional, Protocol
+from typing import TYPE_CHECKING, Any, Dict, List, Optional
 
 if TYPE_CHECKING:
     from .agent import EvoAgent

backend/agents/base/evaluation_hook.py

@@ -8,11 +8,11 @@ from __future__ import annotations
 
 import json
 import logging
-from dataclasses import dataclass, field, asdict
+from dataclasses import dataclass, field
 from datetime import datetime
 from enum import Enum
 from pathlib import Path
-from typing import Any, Dict, List, Optional, Set
+from typing import Any, Dict, List, Optional
 
 logger = logging.getLogger(__name__)
 

backend/agents/base/evo_agent.py

@@ -31,7 +31,6 @@ from .hooks import (
     HOOK_PRE_REASONING,
 )
 from ..prompts.builder import (
-    PromptBuilder,
     build_system_prompt_from_workspace,
 )
 from ..agent_workspace import load_agent_workspace_config

backend/agents/base/skill_adaptation_hook.py

@@ -12,11 +12,10 @@ from dataclasses import dataclass, field
 from datetime import datetime
 from enum import Enum
 from pathlib import Path
-from typing import Any, Dict, List, Optional, Set
+from typing import Any, Dict, List, Optional
 
 from .evaluation_hook import (
     EvaluationCollector,
-    EvaluationResult,
     MetricType,
 )
 

backend/agents/base/tool_guard.py

@@ -12,7 +12,6 @@ from __future__ import annotations
 import asyncio
 import json
 import logging
-from dataclasses import dataclass, field
 from datetime import UTC, datetime
 from enum import Enum