Add manual workflow to clean up branches without an…

Details

A one-shot maintenance workflow (workflow_dispatch only) that deletes
every remote branch that has no open pull request pointing at it. The
default branch and any branch backing an open PR are preserved.
Safety:
- requires the `confirm` input to equal "delete" (prevents accidental runs)
- `dry_run` defaults to true so the first click previews the list
- never runs on push or schedule
Uses the built-in GITHUB_TOKEN (contents:write, pull-requests:read) — no
PAT or new secret required.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Pn8V4h9YWbjYzdSrDiLiVL

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