test: add sanity test for skills installation

Add a sanity test that verifies the skills CLI installation works:
- Runs `npx skills add . -a claude-code -y` using current directory
- Checks command doesn't fail (exit code 0)
- Verifies .claude/skills directory is created
- Dynamically discovers skills in the repo and verifies they're installed
- Checks SKILL.md exists in each installed skill

The test runs on every push to main and pull requests.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -52,3 +52,19 @@ jobs:
             git diff skills/*/AGENTS.md
             exit 1
           fi
+
+  sanity-test:
+    name: Sanity Test (skills add)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+
+      - uses: actions/setup-node@v6
+        with:
+          node-version: '20'
+
+      - name: Install dependencies
+        run: npm install
+
+      - name: Run sanity tests
+        run: npm run test