v0.1.19

Extract auth.py; slim api.py to 836 lines; bump to 0.1.19

- auth.py: _full_login, _refresh_token, _store_aws_credentials,
  _refresh_authentication — all with lazy imports from api.py
- api.py: re-exports auth symbols; down from 1298 → 836 lines across
  three extraction rounds (write_manager, coordinator, auth)
- manifest: 0.1.18 → 0.1.19

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>