feat: add skill validation CI job (#14)

## Summary
- Add skill validation job calling reusable workflow from
skill-repo-skill
- Checks SKILL.md frontmatter, word count, composer.json, plugin.json,
file presence

## Test plan
- [ ] CI passes with new validation job

Author: CybotTM

.github/workflows/lint.yml

@@ -33,3 +33,7 @@ jobs:
           severity: error
         env:
           SHELLCHECK_OPTS: -x
+
+  validate:
+    name: Skill Validation
+    uses: netresearch/skill-repo-skill/.github/workflows/validate.yml@main