Vercel preview cleanup

Authors a GitHub Actions workflow that removes a branch's preview deployments once the branch is gone. This skill installs workflows; it never deletes anything itself. At runtime there is no agent and no model call — deterministic bash + curl only.

Verified API detail (endpoints, pagination, rate limits, retention) lives in reference/vercel-api.md — read it before changing the delete logic.

Step 0 — prerequisites (check before installing)

This skill is downstream of branch deletion. If branches are never deleted, the delete event never fires and this workflow looks broken when it isn't.