fix(ci): use background process instead of jekyll --detach

The --detach flag doesn't work reliably in CI. Use shell backgrounding
with & and wait-on in the same step to ensure server is ready.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: aledlie

.github/workflows/test.yml

@@ -48,10 +48,9 @@ jobs:
         CI: true
 
     - name: Start Jekyll server for performance tests
-      run: bundle exec jekyll serve --detach
-
-    - name: Wait for Jekyll server
-      run: npx wait-on http://localhost:4000 --timeout 60000
+      run: |
+        bundle exec jekyll serve &
+        npx wait-on http://localhost:4000 --timeout 60000
 
     - name: Run performance tests
       run: npm run test:performance