Add data-raw/regenerate_bib.R helper for vignettes/references.bib

[NewGraphEnvironment]

Problem

After v0.2.6, vignettes/references.bib is regenerated by hand
(rbbt::bbt_update_bib(...)) whenever [@key] markers change in
either regional vignette. Easy to forget. Render failures from
unresolved cites show up later than they should.

Scope

Add a small helper to formalize the regen recipe — option 1 of the
three discussed (the conservative one; doesn't break CI):

• New data-raw/regenerate_bib.R — single command that reads both
  regional vignettes, detects the union of [@key] markers, and
  writes vignettes/references.bib via rbbt::bbt_bib(). Documents
  the prerequisites (Zotero running, BBT 9.x for Z8/9) inline.
• HTML comment block at the top of each regional vignette pointing
  to the helper ("after editing cites, run Rscript data-raw/regenerate_bib.R"). HTML comments don't render.
• One-line note in CLAUDE.md (Vignettes section) referencing the
  helper.

Out of scope

• Embedding the rbbt call in YAML at render time (option 2 from the
  discussion) — would break CI which has no Zotero/BBT.
• Pre-commit hook (option 3) — more involved; deferred.

Done when

• data-raw/regenerate_bib.R exists, documented, runnable
• Both vignettes have a top-of-file comment pointing to it
• CLAUDE.md notes the helper
• Running the helper on current main produces no diff (we're at
  18-key parity post-v0.2.6)
• PR + release v0.2.7 (patch — tooling)