Add CHANGELOG.md documenting all implemented features:
- Client with functional options pattern
- Authentication with 2FA and auto token renewal
- HTTP request helper with retry logic
- Data models (Checklist, Task, Note, User, DueDate)
- Error handling with sentinel errors
- Mage build targets
- Comprehensive unit test suite
Follows Keep a Changelog 1.1.0 and Semantic Versioning.
Closes checkvist-api-93m
