refactor(ci): replace staging-deploy with single-repo deploy workflow

Uses peaceiris/actions-gh-pages + GITHUB_TOKEN (no PAT/secret needed).
- push to main → gh-pages:/ (production)
- push to any other branch → gh-pages:/staging/<branch-name>/
keep_files: true on both so staging folders and production coexist.

Pages source switched from main to gh-pages branch.

Author: claudyfaucant

.github/workflows/deploy.yml

@@ -0,0 +1,32 @@
+name: Deploy
+
+on:
+  push:
+    branches:
+      - '**'
+
+permissions:
+  contents: write
+
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Deploy to production
+        if: github.ref_name == 'main'
+        uses: peaceiris/actions-gh-pages@v4
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: .
+          keep_files: true
+
+      - name: Deploy to staging
+        if: github.ref_name != 'main'
+        uses: peaceiris/actions-gh-pages@v4
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: .
+          destination_dir: staging/${{ github.ref_name }}
+          keep_files: true