feat: add Lighthouse CI workflow for quality scoring

Audits 4 pages on every PR: performance, accessibility, best practices,
SEO. Accessibility < 90 blocks merge; others warn at < 90.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: braboj

.github/workflows/lighthouse.yml

@@ -0,0 +1,37 @@
+name: Lighthouse CI
+
+on:
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  lighthouse:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: "22.12.0"
+          cache: npm
+          cache-dependency-path: astro-site/package-lock.json
+
+      - name: Install dependencies
+        working-directory: astro-site
+        run: npm ci
+
+      - name: Create assets symlink
+        run: mkdir -p astro-site/src/content && ln -s ../../../assets astro-site/src/content/assets
+
+      - name: Build site
+        working-directory: astro-site
+        run: npm run build
+
+      - name: Lighthouse CI
+        uses: treosh/lighthouse-ci-action@v12
+        with:
+          configPath: ./lighthouserc.json
+          uploadArtifacts: true

lighthouserc.json

@@ -0,0 +1,24 @@
+{
+  "ci": {
+    "collect": {
+      "staticDistDir": "./astro-site/dist",
+      "url": [
+        "/tutorial-git/",
+        "/tutorial-git/introduction/",
+        "/tutorial-git/building-blocks/",
+        "/tutorial-git/branching-and-merging/"
+      ]
+    },
+    "assert": {
+      "assertions": {
+        "categories:performance": ["warn", { "minScore": 0.9 }],
+        "categories:accessibility": ["error", { "minScore": 0.9 }],
+        "categories:best-practices": ["warn", { "minScore": 0.9 }],
+        "categories:seo": ["warn", { "minScore": 0.9 }]
+      }
+    },
+    "upload": {
+      "target": "temporary-public-storage"
+    }
+  }
+}