fix: add TRADING_DAY_ID write to runtime config and improve test coverage

Changes:
- Write TRADING_DAY_ID to runtime config after creating trading_day record in BaseAgent
- Fix datetime deprecation warnings by replacing datetime.utcnow() with datetime.now(timezone.utc)
- Add test for trading_day_id=None fallback path to verify runtime config lookup works correctly

This ensures trade tools can access trading_day_id from runtime config when not explicitly passed.

agent/base_agent/base_agent.py

@@ -534,6 +534,10 @@ Summary:"""
             days_since_last_trading=pnl_metrics["days_since_last_trading"]
         )
 
+        # Write trading_day_id to runtime config for trade tools
+        from tools.general_tools import write_config_value
+        write_config_value('TRADING_DAY_ID', trading_day_id)
+
         # 6. Run AI trading session
         action_count = 0