fix: update context_injector with trading_day_id after creation

Changes:
- Update context_injector.trading_day_id after trading_day record is created

Root Cause:
- ContextInjector was created before trading_day record existed
- trading_day_id was None when context_injector was initialized
- Even though trading_day_id was written to runtime config, the
  context_injector's attribute was never updated
- MCP tools use the injected trading_day_id parameter, not runtime config

Flow:
1. ModelDayExecutor creates ContextInjector (trading_day_id=None)
2. Agent.run_trading_session() creates trading_day record
3. NEW: Update context_injector.trading_day_id = trading_day_id
4. MCP tools receive trading_day_id via context injection

Impact:
- Fixes: "Trade failed: trading_day_id not found in runtime config"
- Trading tools (buy/sell) can now record actions properly
- Actions are linked to correct trading_day record

Related: agent/base_agent/base_agent.py:541-543

agent/base_agent/base_agent.py

@@ -538,6 +538,10 @@ Summary:"""
         from tools.general_tools import write_config_value
         write_config_value('TRADING_DAY_ID', trading_day_id)
 
+        # Update context_injector with trading_day_id for MCP tools
+        if self.context_injector:
+            self.context_injector.trading_day_id = trading_day_id
+
         # 6. Run AI trading session
         action_count = 0