fix: enable intra-day position tracking for sell-then-buy trades

Resolves issue where sell proceeds were not immediately available for
subsequent buy orders within the same trading session.

Problem:
- Both buy() and sell() independently queried database for starting position
- Multiple trades within same day all saw pre-trade cash balance
- Agents couldn't rebalance portfolios (sell + buy) in single session

Solution:
- ContextInjector maintains in-memory position state during trading session
- Position updates accumulate after each successful trade
- Position state injected into buy/sell via _current_position parameter
- Reset position state at start of each trading day

Changes:
- agent/context_injector.py: Add position tracking with reset_position()
- agent_tools/tool_trade.py: Accept _current_position in buy/sell functions
- agent/base_agent/base_agent.py: Reset position state daily
- tests: Add 13 comprehensive tests for position tracking

All new tests pass. Backward compatible, no schema changes required.

agent/base_agent/base_agent.py

@@ -533,6 +533,8 @@ Summary:"""
         # Update context injector with current trading date
         if self.context_injector:
             self.context_injector.today_date = today_date
+            # Reset position state for new trading day (enables intra-day tracking)
+            self.context_injector.reset_position()
 
         # Clear conversation history for new trading day
         self.clear_conversation_history()