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.

tests/unit/test_context_injector.py

@@ -0,0 +1,192 @@
+"""Test ContextInjector position tracking functionality."""
+
+import pytest
+from agent.context_injector import ContextInjector
+
+
+@pytest.fixture
+def injector():
+    """Create a ContextInjector instance for testing."""
+    return ContextInjector(
+        signature="test-model",
+        today_date="2025-01-15",
+        job_id="test-job-123",
+        trading_day_id=1
+    )
+
+
+class MockRequest:
+    """Mock MCP tool request."""
+    def __init__(self, name, args=None):
+        self.name = name
+        self.args = args or {}
+
+
+async def mock_handler_success(request):
+    """Mock handler that returns a successful position update."""
+    # Simulate a successful trade returning updated position
+    if request.name == "sell":
+        return {
+            "CASH": 1100.0,
+            "AAPL": 7,
+            "MSFT": 5
+        }
+    elif request.name == "buy":
+        return {
+            "CASH": 50.0,
+            "AAPL": 7,
+            "MSFT": 12
+        }
+    return {}
+
+
+async def mock_handler_error(request):
+    """Mock handler that returns an error."""
+    return {"error": "Insufficient cash"}
+
+
+@pytest.mark.asyncio
+async def test_context_injector_initializes_with_no_position(injector):
+    """Test that ContextInjector starts with no position state."""
+    assert injector._current_position is None
+
+
+@pytest.mark.asyncio
+async def test_context_injector_reset_position(injector):
+    """Test that reset_position() clears position state."""
+    # Set some position state
+    injector._current_position = {"CASH": 5000.0, "AAPL": 10}
+
+    # Reset
+    injector.reset_position()
+
+    assert injector._current_position is None
+
+
+@pytest.mark.asyncio
+async def test_context_injector_injects_parameters(injector):
+    """Test that context parameters are injected into buy/sell requests."""
+    request = MockRequest("buy", {"symbol": "AAPL", "amount": 10})
+
+    # Mock handler that just returns the request args
+    async def handler(req):
+        return req.args
+
+    result = await injector(request, handler)
+
+    # Verify context was injected
+    assert result["signature"] == "test-model"
+    assert result["today_date"] == "2025-01-15"
+    assert result["job_id"] == "test-job-123"
+    assert result["trading_day_id"] == 1
+
+
+@pytest.mark.asyncio
+async def test_context_injector_tracks_position_after_successful_trade(injector):
+    """Test that position state is updated after successful trades."""
+    assert injector._current_position is None
+
+    # Execute a sell trade
+    request = MockRequest("sell", {"symbol": "AAPL", "amount": 3})
+    result = await injector(request, mock_handler_success)
+
+    # Verify position was updated
+    assert injector._current_position is not None
+    assert injector._current_position["CASH"] == 1100.0
+    assert injector._current_position["AAPL"] == 7
+    assert injector._current_position["MSFT"] == 5
+
+
+@pytest.mark.asyncio
+async def test_context_injector_injects_current_position_on_subsequent_trades(injector):
+    """Test that current position is injected into subsequent trade requests."""
+    # First trade - establish position
+    request1 = MockRequest("sell", {"symbol": "AAPL", "amount": 3})
+    await injector(request1, mock_handler_success)
+
+    # Second trade - should receive current position
+    request2 = MockRequest("buy", {"symbol": "MSFT", "amount": 7})
+
+    async def verify_injection_handler(req):
+        # Verify that _current_position was injected
+        assert "_current_position" in req.args
+        assert req.args["_current_position"]["CASH"] == 1100.0
+        assert req.args["_current_position"]["AAPL"] == 7
+        return mock_handler_success(req)
+
+    await injector(request2, verify_injection_handler)
+
+
+@pytest.mark.asyncio
+async def test_context_injector_does_not_update_position_on_error(injector):
+    """Test that position state is NOT updated when trade fails."""
+    # First successful trade
+    request1 = MockRequest("sell", {"symbol": "AAPL", "amount": 3})
+    await injector(request1, mock_handler_success)
+
+    original_position = injector._current_position.copy()
+
+    # Second trade that fails
+    request2 = MockRequest("buy", {"symbol": "MSFT", "amount": 100})
+    result = await injector(request2, mock_handler_error)
+
+    # Verify position was NOT updated
+    assert injector._current_position == original_position
+    assert "error" in result
+
+
+@pytest.mark.asyncio
+async def test_context_injector_does_not_inject_position_for_non_trade_tools(injector):
+    """Test that position is not injected for non-buy/sell tools."""
+    # Set up position state
+    injector._current_position = {"CASH": 5000.0, "AAPL": 10}
+
+    # Call a non-trade tool
+    request = MockRequest("search", {"query": "market news"})
+
+    async def verify_no_injection_handler(req):
+        assert "_current_position" not in req.args
+        return {"results": []}
+
+    await injector(request, verify_no_injection_handler)
+
+
+@pytest.mark.asyncio
+async def test_context_injector_full_trading_session_simulation(injector):
+    """Test full trading session with multiple trades and position tracking."""
+    # Reset position at start of day
+    injector.reset_position()
+    assert injector._current_position is None
+
+    # Trade 1: Sell AAPL
+    request1 = MockRequest("sell", {"symbol": "AAPL", "amount": 3})
+
+    async def handler1(req):
+        # First trade should NOT have injected position
+        assert req.args.get("_current_position") is None
+        return {"CASH": 1100.0, "AAPL": 7}
+
+    result1 = await injector(request1, handler1)
+    assert injector._current_position == {"CASH": 1100.0, "AAPL": 7}
+
+    # Trade 2: Buy MSFT (should use position from trade 1)
+    request2 = MockRequest("buy", {"symbol": "MSFT", "amount": 7})
+
+    async def handler2(req):
+        # Second trade SHOULD have injected position from trade 1
+        assert req.args["_current_position"]["CASH"] == 1100.0
+        assert req.args["_current_position"]["AAPL"] == 7
+        return {"CASH": 50.0, "AAPL": 7, "MSFT": 7}
+
+    result2 = await injector(request2, handler2)
+    assert injector._current_position == {"CASH": 50.0, "AAPL": 7, "MSFT": 7}
+
+    # Trade 3: Failed trade (should not update position)
+    request3 = MockRequest("buy", {"symbol": "GOOGL", "amount": 100})
+
+    async def handler3(req):
+        return {"error": "Insufficient cash", "cash_available": 50.0}
+
+    result3 = await injector(request3, handler3)
+    # Position should remain unchanged after failed trade
+    assert injector._current_position == {"CASH": 50.0, "AAPL": 7, "MSFT": 7}