feat: remove /reasoning endpoint (replaced by /results)

- Delete Pydantic models: ReasoningMessage, PositionSummary, TradingSessionResponse, ReasoningResponse
- Delete /reasoning endpoint from api/main.py
- Remove /reasoning documentation from API_REFERENCE.md
- Delete old endpoint tests (test_api_reasoning_endpoint.py)
- Add integration tests verifying /results replaces /reasoning

The /reasoning endpoint has been replaced by /results with reasoning parameter:
- GET /reasoning?job_id=X -> GET /results?job_id=X&reasoning=summary
- GET /reasoning?job_id=X&include_full_conversation=true -> GET /results?job_id=X&reasoning=full

Benefits of new endpoint:
- Day-centric structure (easier to understand portfolio progression)
- Daily P&L metrics included
- AI-generated reasoning summaries
- Unified data model (trading_days, actions, holdings)

tests/integration/test_results_replaces_reasoning.py

@@ -0,0 +1,100 @@
+"""Verify /results endpoint replaces /reasoning endpoint."""
+
+import pytest
+from fastapi.testclient import TestClient
+from api.main import create_app
+from api.database import Database
+
+
+def test_results_with_full_reasoning_replaces_old_endpoint(tmp_path):
+    """Test /results?reasoning=full provides same data as old /reasoning."""
+
+    # Create test database with file path (not in-memory, to avoid sharing issues)
+    import json
+    db_path = str(tmp_path / "test.db")
+    db = Database(db_path)
+
+    # Create job first
+    db.connection.execute("""
+        INSERT INTO jobs (job_id, config_path, status, date_range, models, created_at)
+        VALUES (?, ?, ?, ?, ?, ?)
+    """, ('test-job-123', 'test_config.json', 'completed',
+          json.dumps({'init_date': '2025-01-15', 'end_date': '2025-01-15'}),
+          json.dumps(['test-model']), '2025-01-15T10:00:00Z'))
+    db.connection.commit()
+
+    trading_day_id = db.create_trading_day(
+        job_id='test-job-123',
+        model='test-model',
+        date='2025-01-15',
+        starting_cash=10000.0,
+        starting_portfolio_value=10000.0,
+        ending_cash=8500.0,
+        ending_portfolio_value=10000.0,
+        daily_profit=0.0,
+        daily_return_pct=0.0,
+        days_since_last_trading=0
+    )
+
+    # Add actions
+    db.create_action(trading_day_id, 'buy', 'AAPL', 10, 150.0)
+
+    # Add holdings
+    db.create_holding(trading_day_id, 'AAPL', 10)
+
+    # Update with reasoning
+    db.connection.execute("""
+        UPDATE trading_days
+        SET reasoning_summary = 'Bought AAPL based on earnings',
+            reasoning_full = ?,
+            total_actions = 1
+        WHERE id = ?
+    """, (json.dumps([
+        {"role": "user", "content": "System prompt"},
+        {"role": "assistant", "content": "I will buy AAPL"}
+    ]), trading_day_id))
+
+    db.connection.commit()
+    db.connection.close()
+
+    # Create test app with the test database
+    app = create_app(db_path=db_path)
+    app.state.test_mode = True
+
+    # Override the database dependency to use our test database
+    from api.routes.results_v2 import get_database
+
+    def override_get_database():
+        return Database(db_path)
+
+    app.dependency_overrides[get_database] = override_get_database
+
+    client = TestClient(app)
+
+    # Query new endpoint
+    response = client.get("/results?job_id=test-job-123&reasoning=full")
+
+    assert response.status_code == 200
+    data = response.json()
+
+    # Verify structure matches old endpoint needs
+    assert data['count'] == 1
+    result = data['results'][0]
+
+    assert result['date'] == '2025-01-15'
+    assert result['model'] == 'test-model'
+    assert result['trades'][0]['action_type'] == 'buy'
+    assert result['trades'][0]['symbol'] == 'AAPL'
+    assert isinstance(result['reasoning'], list)
+    assert len(result['reasoning']) == 2
+
+
+def test_reasoning_endpoint_returns_404():
+    """Verify /reasoning endpoint is removed."""
+
+    app = create_app(db_path=":memory:")
+    client = TestClient(app)
+
+    response = client.get("/reasoning?job_id=test-job-123")
+
+    assert response.status_code == 404