ci: add GitHub Actions workflow for build and deploy

Add a GitHub Actions workflow that builds the Astro site and deploys
to GitHub Pages using the withastro/action.

- PR to main: build only (validates the site compiles)
- Push to main (merged PR): build + deploy
- workflow_dispatch: manual build + deploy
- Concurrency control cancels in-progress deploys
- Minimal permissions: contents:read for build, pages:write and
  id-token:write only for the deploy job

Signed-off-by: David Justice <david@justice.dev>

Author: devigned

.github/workflows/deploy.yml

@@ -0,0 +1,43 @@
+name: Build & Deploy Site
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: pages-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  build:
+    name: Build
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Build with Astro
+        uses: withastro/action@v3
+
+  deploy:
+    name: Deploy
+    if: github.event_name == 'push' || github.event_name == 'workflow_dispatch'
+    needs: build
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pages: write
+      id-token: write
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4