ci(site): add format/lint/test/build workflow for astro site

Add Site GitHub Actions workflow running prettier format check, astro
check, tests, and build on changes under site/. Wire prettier into the
site package with config, ignore, and scripts, and format existing
sources.

.github/workflows/site.yml

@@ -0,0 +1,43 @@
+name: Site
+
+on:
+  pull_request:
+    paths:
+      - "site/**"
+      - ".github/workflows/site.yml"
+  push:
+    branches: [ main ]
+    paths:
+      - "site/**"
+      - ".github/workflows/site.yml"
+
+permissions:
+  contents: read
+
+defaults:
+  run:
+    working-directory: site
+
+jobs:
+  build-site:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6
+        with:
+          persist-credentials: false
+      - uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020 # v4.4.0
+        with:
+          node-version: 24
+          cache: npm
+          cache-dependency-path: site/package-lock.json
+      - name: Install dependencies
+        run: npm ci
+      - name: Format
+        run: npm run format:check
+      - name: Lint
+        run: npm run check
+      - name: Test
+        run: npm test --if-present
+      - name: Build
+        run: npm run build