ai(claude[commands]) Add /update-libvcs skill

why: Codify the libvcs dependency update workflow as a reusable,
gold-standard command with exemplar commit patterns from real history.
what:
- Add 7-phase workflow: discovery, package bump, code adaptation,
  test adaptation, CHANGES entry, verification, PR
- Inline commit pattern exemplars from vcspull git history (3 tiers)
- Support --dry-run and --no-pr flags

Author: tony