Enforce Agent Skills name validation on the sync write path, not only under --validate

[sebastientaggart]

Follow-up auto-proposed from the code review on PR #220.

Original finding:

[WARNING] Spec-name validation runs only under --validate; the write path and CI's --dry-run don't catch a frontmatter-name/directory mismatch, so noncompliant output could ship to consumer repos undetected.

Failure scenario: a skill's frontmatter name is renamed without renaming its directory (or vice versa) — sync_skill silently writes output under fm['name'], CI's ./sync.py --dry-run passes, and name/directory-mismatched output lands in every consumer repo until someone manually runs --validate.

Possible directions (to be decided at implementation time): run validate_skill_names as part of the normal sync/dry-run path, or add --validate to the CI sync-check workflow alongside --dry-run.

See the review comment thread on #220 for context.