Delete unchanged merged branches safely

[Scriptception]

Summary

• remove same-repository branches after their pull requests merge into main
• backfill cleanup for existing merged pull requests on the next main push
• preserve forks, open pull request heads, advanced branches, and protected branches with exact-SHA leases

Validation

• workflow YAML and embedded Bash syntax validated
• deletion and preservation paths exercised with mocked remote/API responses
• action dependency pinned to an immutable commit