chore(ci): record the demo alongside test and deploy

[sam-goodwin]

Stacked on #9.

The demo job waited for test and deploy (~6 min), but it deploys its own stages (demo-, dev-, test-demo-pr-<n>) and only needed the preview URL for its comment.

before: check → test → deploy → demo (comment)
after:  check → test → deploy ─┐
             └→ demo ──────────┴→ demo-comment

  demo:
    needs: check
    outputs:
      url: ${{ steps.upload.outputs.url }}

  demo-comment:
    if: github.event_name == 'pull_request'
    needs: [deploy, demo]
    runs-on: ubuntu-latest
    steps:
      - run: bun packages/demos/comment.ts
        env:
          DEMO_URL: ${{ needs.demo.outputs.url }}
          APP_URL: ${{ needs.deploy.outputs.url }}

The macOS runner queue still applies, but it now overlaps with test and deploy instead of following them.

🤖 Generated with Claude Code

[github-actions]

Preview deployed

App: https://cafe-web-pr-10-szezdpjidwn3zqed.testing-2b2.workers.dev

Built from commit 8063e85.

---

This comment updates automatically with each push.

[github-actions]

Demo of this preview

Video: https://pub-68e74efbba3b4272943c198da363bcab.r2.dev/pr-10/3404b26cf05b76774fe63dde2e1ccec08d6a2f10/demo.mp4
App: https://cafe-web-pr-10-szezdpjidwn3zqed.testing-2b2.workers.dev

Recorded from commit 3404b26: alchemy deploy, chatting in the browser, alchemy dev, pnpm test, ALCHEMY_DEV=1 pnpm test.