CF11-004: Execution Storage Schema (Phase 11)
- Add TypeScript types for executions, steps, queries, and stats
- Add SQLite schema with workflow_executions and execution_steps tables
- Implement ExecutionStore class with full CRUD operations
- Support query filtering by workflow, status, trigger type, date range
- Support pagination with limit/offset
- Add retention utilities (cleanup by age, count, total)
- Add aggregated statistics (success rate, avg duration)
- Add unit tests with MockDatabase for all operations
Uses synchronous SQLite (better-sqlite3 compatible interface) for
performant local storage of execution history data.
