Branch cleanup

Authors GitHub Actions workflows that delete stale branches. This skill installs workflows; it never deletes anything itself. At runtime there is no agent and no model call — deterministic bash + gh only.

Verified GitHub behaviour (the delete event, ruleset errors, gh footguns, base-branch semantics) lives in reference/github-facts.md. Read it before changing any deletion logic.

The one rule that must never be relaxed

Before deleting ANY branch, check for open PRs that target it as base:

gh pr list --base <branch> --state open --json number --limit 1000