Switch to modern GitHub Pages deployment via Actions

Replace peaceiris/actions-gh-pages (branch-based deploy) with the
official actions/upload-pages-artifact + actions/deploy-pages workflow.
The old approach required a gh-pages branch that was never created,
causing GitHub Pages to serve the raw README instead of the built site.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jnbdz

.github/workflows/deploy.yml

@@ -6,10 +6,16 @@ on:
       - master
 
 permissions:
-  contents: write
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: "pages"
+  cancel-in-progress: false
 
 jobs:
-  deploy:
+  build:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
@@ -36,8 +42,18 @@ jobs:
       - name: Copy CNAME to build output
         run: cp CNAME site/CNAME
 
-      - name: Deploy to GitHub Pages
-        uses: peaceiris/actions-gh-pages@v4
+      - name: Upload artifact
+        uses: actions/upload-pages-artifact@v3
         with:
-          github_token: ${{ secrets.GITHUB_TOKEN }}
-          publish_dir: ./site
+          path: ./site
+
+  deploy:
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    runs-on: ubuntu-latest
+    needs: build
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4