Move provider auth and OAuth flows onto pi-ai Models, compose models.json and extension overlays through ModelRuntime, and retain ModelRegistry as an extension compatibility facade.
- Add src/cli.ts with login command for OAuth providers
- Add bin entry to package.json for 'npx @mariozechner/pi-ai'
- Update README: remove setApiKey docs, rewrite OAuth section
- OAuth storage is caller's responsibility, not library's
- Use getOAuthProviders() instead of duplicating provider list