Formalise the contribution model instead of moving generation into CI (which would
need PlantUML on the runners). Contributors edit only the ttl; a maintainer
regenerates and publishes.

- Add `npm run regen`: validate-vocab.py, then ttl-to-puml.py (diagrams) and
  build-migration-ledger.py (ledger) — the single "after an ontology change" step.
- README: a "Contributing changes (the DMR process)" section (ticket -> ttl MR ->
  maintainer regen/publish) and a "Maintainer setup" section with the full toolchain,
  including the PlantUML + Java + Graphviz install (brew and apt) that diagram
  regeneration needs, plus the regenerate-and-publish runbook.
- CONTRIBUTING.md: a short GitLab-surfaced front door pointing at the README.
- CLAUDE.md: correct the generated-files list (ontology/*/docs and the migration
  ledger are generated but COMMITTED, not git-ignored) and point the post-change
  step at `npm run regen`.

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