Build change-log pages in CI + auto-tag main…

Details

- ci.yml / pr-validation.yml / preview-v3.yml / deploy-vercel.yml:
  check out with fetch-depth: 0 and run `npm run changelog:generate`
  before the Jekyll build, so every built site carries the full
  changelog derived from history.
- New version-tag.yml: on each push to main, verify the version
  invariants and push the computed annotated tag (vX.Y.Z) — production
  tags continue the retroactive sequence exactly. Creates tags only;
  skips silently when the tag exists.
- CLAUDE.md: document the generate step in the local build sequence.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Mn7fa3Z4bpbSAj4edBnjXA

Files changed (6)