🛠️ Repo: Automatically remove stale branches

[mark-wiemer]

Tooling Suggestion Checklist

• I have tried restarting my IDE and the issue persists.
• I have pulled the latest main branch of the repository.
• I have read and agree to Mocha's Code of Conduct and Contributing Guidelines
• I have searched for related issues and issues with the faq label, but none matched my issue.
• I want to provide a PR to resolve this

Overview

Mark (that's me!) decided to use mochajs/mocha directly for repros and bugfix branches, and Mark isn't so good at cleaning up after himself.

We should work to minimize branches created directly in this repo, and instead guide everyone, maintainers and AI agents, to create branches on forks unless there's good reason to create them here (e.g. stress-testing changes faster with our larger GitHub Actions worker pool).

We should also remove the existing branches that we've kept for PR reference, carefully replacing them with fork branches and updating all relevant references.

Finally, we should add a bot to automatically ping maintainers if a branch has been unmodified for 7 days but isn't associated with an open PR.

Additional Info

Let's keep this issue open until the only two branches on mochajs/mocha are main and 11.x (and other irreplaceable branches, if it takes that long)