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_stock_handlers.py

@@ -15,7 +15,6 @@ from backend.domains import trading as trading_domain
 from backend.enrich.news_enricher import enrich_news_for_symbol
 from backend.enrich.llm_enricher import llm_enrichment_enabled
 from backend.tools.data_tools import prices_to_df
-from shared.client import NewsServiceClient, TradingServiceClient
 
 logger = logging.getLogger(__name__)
 
@@ -564,7 +563,6 @@ async def handle_get_stock_technical_indicators(gateway: Any, websocket: Any, da
         df = prices_to_df(prices)
         signal = gateway._technical_analyzer.analyze(ticker, df)
 
-        import pandas as pd
         df_sorted = df.sort_values("time").reset_index(drop=True)
         df_sorted["returns"] = df_sorted["close"].pct_change()
         vol_10 = float(df_sorted["returns"].tail(10).std() * (252**0.5) * 100) if len(df_sorted) >= 10 else None