Add scheme preview deploy workflow

Author: Ronan-R-R

.github/workflows/previews.yml

@@ -0,0 +1,66 @@
+name: Scheme previews
+
+# Builds the current production site (main) at the root and hosts the four
+# redesign colour schemes under /preview/<scheme>/ so they can be compared side
+# by side. The live site at the root stays exactly as main produces it.
+
+on:
+  push:
+    branches: [redesign/previews]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: pages
+  cancel-in-progress: false
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout main (root site)
+        uses: actions/checkout@v4
+        with:
+          ref: main
+          path: root
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+
+      - name: Build root site (production, unchanged)
+        working-directory: root
+        run: |
+          npm ci
+          npm run build
+          mkdir -p "$GITHUB_WORKSPACE/_site"
+          cp -r dist/. "$GITHUB_WORKSPACE/_site/"
+
+      - name: Build scheme previews
+        run: |
+          for s in cyan-ink ember light-editorial mono-lime; do
+            rm -rf scheme
+            git clone --depth 1 --branch "redesign/$s" \
+              "https://x-access-token:${{ github.token }}@github.com/${{ github.repository }}.git" scheme
+            (cd scheme && npm ci && PREVIEW_BASE="/preview/$s/" npm run build)
+            mkdir -p "$GITHUB_WORKSPACE/_site/preview/$s"
+            cp -r scheme/dist/. "$GITHUB_WORKSPACE/_site/preview/$s/"
+          done
+
+      - uses: actions/upload-pages-artifact@v3
+        with:
+          path: _site
+
+  deploy:
+    needs: build
+    runs-on: ubuntu-latest
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - id: deployment
+        uses: actions/deploy-pages@v4