Add /bib-update skill for idempotent references.bib maintenance

[nsmiller2501]

Summary

This PR adds /bib-update, a skill for assembling and refreshing a project-level references/references.bib file from per-paper metadata blocks.

What changed

• Adds /bib-update instructions for append-only BibTeX maintenance.
• Reads ## Bibliographic metadata blocks from _text.md extracts created by /split-pdf, /read-pdf, or /wiki-update.
• Uses a DOI → CrossRef → OpenAlex → approved fallback cascade.
• Bootstraps missing metadata or bare PDFs from page 1 when possible.
• Keeps default mode idempotent by skipping citation keys already present in references/references.bib; --rebuild-bib is the explicit regeneration path.
• Adds the skill to the visible skills index and frames it as a companion to /bibcheck, not a replacement.

Testing

• Reviewed the branch diff against the PR notes.
• Re-audited skip/idempotency behavior and complement-to-/bibcheck framing.
• No automated tests were run; this is a skill/documentation protocol update.