chore(ci): run PR cleanup in its own workflow so it is never cancelled

[sam-goodwin]

Cleanup lived in ci.yml, inside the per-PR concurrency group with cancel-in-progress: true:

• a newer run in that group could cancel cleanup mid-destroy, and
• the closed run itself cancelled the PR's in-progress CI run, so Test.make's afterAll(destroy) and the demo teardown never ran.

Either way, stages were left behind.

cleanup.yml: its own workflow, never cancelled

on:
  pull_request:
    types: [closed]

concurrency:
  group: cleanup-pr-${{ github.event.number }}
  cancel-in-progress: false

It waits for the PR's CI runs to finish (so nothing deploys after the destroy, and the test suites clean up after themselves), then destroys:

gh api ".../workflows/ci.yml/runs?event=pull_request&branch=$HEAD_REF" \
  --jq '[.workflow_runs[] | select(.status != "completed")] | length'   # poll until 0 (max 60 min)

pnpm exec alchemy destroy --stage pr-$N --yes
pnpm exec alchemy destroy --stage demo-pr-$N --yes
pnpm exec alchemy destroy --stage dev-pr-$N --yes

ci.yml no longer handles closed

-    types: [opened, reopened, synchronize, closed]
+    types: [opened, reopened, synchronize]

Superseded pushes still cancel in-progress PR runs. They reuse the same test-pr-<n> / demo-pr-<n> stages, so the next run deploys over anything the cancelled run left and tears it down at the end.

🤖 Generated with Claude Code

[github-actions]

Preview deployed

App: https://cafe-web-pr-9-3altvykwkdrowm4o.testing-2b2.workers.dev

Built from commit 53ada1b.

---

This comment updates automatically with each push.

[github-actions]

Demo of this preview

Video: https://pub-68e74efbba3b4272943c198da363bcab.r2.dev/pr-9/f45c77a7e77ac538d445eb43c3969d00ed24a33c/demo.mp4
App: https://cafe-web-pr-9-3altvykwkdrowm4o.testing-2b2.workers.dev

Recorded from commit f45c77a: alchemy deploy, chatting in the browser, alchemy dev, pnpm test, ALCHEMY_DEV=1 pnpm test.