Slim borg tier 2: nightly workers.dev run with cost guard rails #866

Split out of #855. Slim borg's tier 1 (local twins, desktop and box hosts) runs on every PR. Tier 2 is the same suite against a real workers.dev deploy.

• A workflow, nightly or on a borg label: rustybuns plan, deploy, bun borg.ts <url> --limits, destroy, and publish the grid (job summary is enough to start).
• Guard rails for any run that costs money:
  • a cost cap read from plan: refuse to deploy over it
  • destroy runs even when the checks fail (if: always())
  • a sweeper that removes anything tagged borg older than an hour, for runs that died before their destroy
• The owner adds a Cloudflare API token as a CI secret (agents can't). The workflow should sit behind an environment that needs the owner's approval, like npm-publish.yml.

Blocked on the owner for the secret, and on #865 for knowing which account plan the run needs.