feat: implement reasoning logs API with database-only storage

Complete implementation of reasoning logs retrieval system that
replaces JSONL file-based logging with database-only storage.

Database Changes:
- Add trading_sessions table (one record per model-day)
- Add reasoning_logs table (conversation history with summaries)
- Add session_id column to positions table
- Add indexes for query performance

Agent Changes:
- Add conversation history tracking to BaseAgent
- Add AI-powered summary generation using same model
- Remove JSONL logging code (_log_message, _setup_logging)
- Preserve in-memory conversation tracking

ModelDayExecutor Changes:
- Create trading session at start of execution
- Store reasoning logs with AI-generated summaries
- Update session summary after completion
- Link positions to sessions via session_id

API Changes:
- Add GET /reasoning endpoint with filters (job_id, date, model)
- Support include_full_conversation parameter
- Return both summaries and full conversation on demand
- Include deployment mode info in responses

Documentation:
- Add complete API reference for GET /reasoning
- Add design document with architecture details
- Add implementation guide with step-by-step tasks
- Update Python and TypeScript client examples

Testing:
- Add 6 tests for conversation history tracking
- Add 4 tests for summary generation
- Add 5 tests for model_day_executor integration
- Add 8 tests for GET /reasoning endpoint
- Add 9 integration tests for E2E flow
- Update existing tests for schema changes

All 32 new feature tests passing. Total: 285 tests passing.

.gitignore

@@ -63,12 +63,8 @@ configs/hour_config.json
 configs/test_config.json
 configs/test_day_config.json
 
-# Data directories (optional - uncomment if needed)
-data/agent_data/test*/
-data/agent_data/*test*/
-data/dev_agent_data/
-data/merged_daily.jsonl
-data/merged_hour.jsonl
+# Data directories
+data/
 
 # Jupyter Notebook
 .ipynb_checkpoints