chore(ci): make skill validation blocking (#392)

All 55 skills now comply with SKILL-FORMAT.md, so skill validation
can be a required check that blocks PRs with non-compliant skills.

Refs: #390

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: mcj-coder

.github/workflows/ci.yml

@@ -45,7 +45,6 @@ jobs:
   skill-validation:
     name: Skill Structure Validation
     runs-on: ubuntu-latest
-    continue-on-error: true # TODO: Remove once skills comply with SKILL-FORMAT.md
     steps:
       - uses: actions/checkout@v4