ci: add validate:skills to GitHub quality workflow

The deterministic skill validator was in the npm quality chain but
missing from the GitHub Actions workflow.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/quality.yaml

@@ -108,3 +108,6 @@ jobs:
 
       - name: Validate file references
         run: npm run validate:refs
+
+      - name: Validate skills
+        run: npm run validate:skills