Add Surge.sh PR preview deployments

- On PR open/push: builds site, deploys to petrsvihlik-pr-{N}.surge.sh,
  posts (or updates) a comment on the PR with the preview URL
- On PR close: tears down the Surge deployment automatically

Requires SURGE_LOGIN and SURGE_TOKEN repository secrets.

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

Author: petrsvihlik

.github/workflows/preview.yml

@@ -0,0 +1,69 @@
+name: Preview
+
+on:
+  pull_request:
+    branches: [master]
+    types: [opened, synchronize, reopened, closed]
+
+jobs:
+  deploy-preview:
+    if: github.event.action != 'closed'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+      - uses: actions/setup-dotnet@v5
+        with:
+          dotnet-version: 10.0.x
+      - name: Install dependencies
+        run: dotnet restore
+      - name: Build
+        run: dotnet build --configuration Release --no-restore
+      - name: Generate site
+        run: dotnet run --configuration Release
+      - name: Index with Pagefind
+        run: npx pagefind --site output
+      - name: Deploy to Surge
+        env:
+          SURGE_LOGIN: ${{ secrets.SURGE_LOGIN }}
+          SURGE_TOKEN: ${{ secrets.SURGE_TOKEN }}
+        run: npx surge ./output petrsvihlik-pr-${{ github.event.pull_request.number }}.surge.sh
+      - name: Post preview URL comment
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const url = 'https://petrsvihlik-pr-${{ github.event.pull_request.number }}.surge.sh';
+            const marker = '<!-- surge-preview -->';
+            const body = `${marker}\n🚀 **Preview:** [${url}](${url})`;
+
+            const comments = await github.rest.issues.listComments({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              issue_number: context.issue.number,
+            });
+
+            const existing = comments.data.find(c => c.body.includes(marker));
+            if (existing) {
+              await github.rest.issues.updateComment({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                comment_id: existing.id,
+                body,
+              });
+            } else {
+              await github.rest.issues.createComment({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                issue_number: context.issue.number,
+                body,
+              });
+            }
+
+  teardown-preview:
+    if: github.event.action == 'closed'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Teardown Surge preview
+        env:
+          SURGE_LOGIN: ${{ secrets.SURGE_LOGIN }}
+          SURGE_TOKEN: ${{ secrets.SURGE_TOKEN }}
+        run: npx surge teardown petrsvihlik-pr-${{ github.event.pull_request.number }}.surge.sh