feat: finalize library with ETL exports and entity relationship examples
- Add entity relationships example showing data flow patterns
- Create schema export utilities for ETL pipeline generation
- Simplify README to focus on API wrapper functionality
- Generate machine-readable endpoint and ETL mapping schemas
- Remove ETL-specific ERD (moved to separate mlb-etl-datamart project)
- All tests passing (98 unit tests, 39 BDD scenarios)