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/gateway_server.py

@@ -19,16 +19,10 @@ from dotenv import load_dotenv
 # Load environment variables
 load_dotenv()
 
-from backend.agents import AnalystAgent, PMAgent, RiskAgent
-from backend.agents.skills_manager import SkillsManager
-from backend.agents.toolkit_factory import create_agent_toolkit, load_agent_profiles
 from backend.agents.prompt_loader import get_prompt_loader
-from backend.agents.workspace_manager import WorkspaceManager
-from backend.config.constants import ANALYST_TYPES
 from backend.core.pipeline import TradingPipeline
-from backend.core.pipeline_runner import create_agents, create_long_term_memory
+from backend.core.pipeline_runner import create_agents
 from backend.core.scheduler import BacktestScheduler, Scheduler
-from backend.llm.models import get_agent_formatter, get_agent_model
 from backend.runtime.manager import (
     TradingRuntimeManager,
     set_global_runtime_manager,