Add frontmatter and structure validator (CI)

Python validator derived from READ, WRITE, DO, and Entry. Enforces
frontmatter shape, required sections, knowledge-file length and
no-code-blocks rule, sample-sibling naming (<slug>.good.al /
<slug>.bad.al), action-skill section ordering, and unique skill ids
per kind. Runs in GitHub Actions on PRs and pushes to main; emits
GitHub annotations when GITHUB_ACTIONS is set, plain text otherwise.
Warnings do not fail the build.

Passes cleanly against the existing microsoft-layer corpus.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/validate-frontmatter.yml

@@ -0,0 +1,25 @@
+name: Validate frontmatter and structure
+
+on:
+  pull_request:
+    branches: [main]
+  push:
+    branches: [main]
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Install dependencies
+        run: pip install pyyaml
+
+      - name: Run validator
+        run: python .github/scripts/validate_frontmatter.py --root .