ci: deploy live gallery to GitHub Pages and fix README link

The README's "Browse the live gallery" link pointed at the raw HTML blob,
which GitHub renders as source code rather than the gallery. Add a Pages
deployment workflow that publishes skills/diagram-design/assets as a static
site, and point the README at the deployed gallery (closes #33).

.github/workflows/pages.yml

@@ -0,0 +1,41 @@
+name: Deploy gallery to GitHub Pages
+
+on:
+  push:
+    branches: [ main ]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: pages
+  cancel-in-progress: false
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+
+      - name: Configure Pages
+        uses: actions/configure-pages@v6
+
+      - name: Upload gallery artifact
+        uses: actions/upload-pages-artifact@v5
+        with:
+          path: skills/diagram-design/assets
+
+  deploy:
+    needs: build
+    runs-on: ubuntu-latest
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v5