docs: update CLAUDE.md to reflect Text2SQL engine implementation

Update project documentation to reflect Issue #4 completion:

- Update current state to show engine is implemented
- Add Text2SQL engine section with usage examples
- Document key components (QueryIntent, SQLValidator, etc.)
- Update "What's Implemented" section
- Mark Issue #4 as completed
- Update next priority to Issue #18 (smolagents)
- Add engine test command example

Author: Sakeeb91