Drop the unused GitChanges import in api.py. Expand the .gitignore
comment on .contextzip/ to note it's project-local and that the
Gemini API key is never stored there.
- Documents --prompt flag, AI file selection, first-run onboarding
- Adds config subcommand to usage table and examples
- Adds httpx to dependencies in pyproject.toml
- Updates project structure to include ai/ package