Default API URL

Tag-driven releases; polish PyPI metadata and README

- Derive the package version from the git tag via hatch-vcs: __version__
  reads it back from installed distribution metadata, CI fetches full
  history so the tag is visible, and publish.yml guards against shipping a
  dev/local version, a tag/version mismatch, or a version already on PyPI.
- Drop the Tinker mention from the PyPI project description.
- Trim the README Authentication section to a single API-key note.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>