Release + citation: GitHub release + Zenodo DOI + CITATION.cff

[dlebauer]

Goal

• Make the package and data citable, and support versioned dataset snapshots.

Tasks

• Add citation metadata:
• CITATION.cff (GitHub-readable)
  • inst/CITATION (R-readable)
• Configure Zenodo integration for DOI per GitHub release (link repo to Zenodo).

Acceptance criteria

• A user can cite the dataset via the Zenodo DOI for a release.
• citation("betydata") returns a sensible citation.
• CITATION.cff is present and populated.

Non-goals

• No CRAN submission in MVP (attempt can be Phase 2 if desired).

[dlebauer]

I've enabled the hook in Zenodo, so a doi should be generated on the first and subsequent releases.

[dlebauer]

fixed by #12