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.

tests/unit/test_trade_tools_new_schema.py

@@ -139,6 +139,87 @@ def test_buy_writes_to_actions_table(test_db, monkeypatch):
     assert cursor.fetchone() is None, "Old positions table should not exist"
 
 
+def test_buy_with_none_trading_day_id_reads_from_config(test_db, monkeypatch):
+    """Test buy() with trading_day_id=None fallback reads from runtime config."""
+    db, trading_day_id = test_db
+
+    # Create a mock connection wrapper that doesn't actually close
+    class MockConnection:
+        def __init__(self, real_conn):
+            self.real_conn = real_conn
+
+        def cursor(self):
+            return self.real_conn.cursor()
+
+        def execute(self, *args, **kwargs):
+            return self.real_conn.execute(*args, **kwargs)
+
+        def commit(self):
+            return self.real_conn.commit()
+
+        def rollback(self):
+            return self.real_conn.rollback()
+
+        def close(self):
+            pass  # Don't actually close the connection
+
+    mock_conn = MockConnection(db.connection)
+
+    # Mock get_db_connection to return our mock connection
+    monkeypatch.setattr('agent_tools.tool_trade.get_db_connection',
+                       lambda x: mock_conn)
+
+    # Mock get_current_position_from_db to return starting position
+    monkeypatch.setattr('agent_tools.tool_trade.get_current_position_from_db',
+                       lambda job_id, sig, date: ({'CASH': 10000.0}, 0))
+
+    # Mock runtime config
+    monkeypatch.setenv('RUNTIME_ENV_PATH', '/tmp/test_runtime_fallback.json')
+
+    # Create mock runtime config file with TRADING_DAY_ID
+    import json
+    with open('/tmp/test_runtime_fallback.json', 'w') as f:
+        json.dump({
+            'TODAY_DATE': '2025-01-15',
+            'SIGNATURE': 'test-model',
+            'JOB_ID': 'test-job-123',
+            'TRADING_DAY_ID': trading_day_id
+        }, f)
+
+    # Mock price data
+    monkeypatch.setattr('agent_tools.tool_trade.get_open_prices',
+                       lambda date, symbols: {'AAPL_price': 150.0})
+
+    # Execute buy with trading_day_id=None to force config lookup
+    result = _buy_impl(
+        symbol='AAPL',
+        amount=10,
+        signature='test-model',
+        today_date='2025-01-15',
+        job_id='test-job-123',
+        trading_day_id=None  # Force fallback to runtime config
+    )
+
+    # Check if there was an error
+    if 'error' in result:
+        print(f"Buy failed with error: {result}")
+
+    # Verify action record created with correct trading_day_id from config
+    cursor = db.connection.execute("""
+        SELECT action_type, symbol, quantity, price, trading_day_id
+        FROM actions
+        WHERE trading_day_id = ?
+    """, (trading_day_id,))
+
+    row = cursor.fetchone()
+    assert row is not None, "Action record should exist when reading trading_day_id from config"
+    assert row[0] == 'buy'
+    assert row[1] == 'AAPL'
+    assert row[2] == 10
+    assert row[3] == 150.0
+    assert row[4] == trading_day_id, "trading_day_id should match the value from runtime config"
+
+
 def test_sell_writes_to_actions_table(test_db, monkeypatch):
     """Test sell() writes action record to actions table."""
     db, trading_day_id = test_db