Add CI workflow to validate YAML frontmatter in PRs

Adds a GitHub Actions workflow that validates frontmatter in agent,
skill, and command .md files changed by a PR. Checks:

- Agents: name and description are present and parseable
- Skills: description is present (required for Skill tool discovery)
- Commands: description is present and parseable

The workflow only runs when PRs touch files in agents/, skills/, or
commands/ directories, and only validates the changed files.

.github/workflows/validate-frontmatter.yml

@@ -0,0 +1,34 @@
+name: Validate Frontmatter
+
+on:
+  pull_request:
+    paths:
+      - '**/agents/*.md'
+      - '**/skills/*/SKILL.md'
+      - '**/commands/*.md'
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: oven-sh/setup-bun@v2
+
+      - name: Install dependencies
+        run: cd .github/scripts && bun install yaml
+
+      - name: Get changed frontmatter files
+        id: changed
+        run: |
+          FILES=$(gh pr diff ${{ github.event.pull_request.number }} --name-only | grep -E '(agents/.*\.md|skills/.*/SKILL\.md|commands/.*\.md)$' || true)
+          echo "files<<EOF" >> "$GITHUB_OUTPUT"
+          echo "$FILES" >> "$GITHUB_OUTPUT"
+          echo "EOF" >> "$GITHUB_OUTPUT"
+        env:
+          GH_TOKEN: ${{ github.token }}
+
+      - name: Validate frontmatter
+        if: steps.changed.outputs.files != ''
+        run: |
+          echo "${{ steps.changed.outputs.files }}" | xargs bun .github/scripts/validate-frontmatter.ts