🛠 Repo: Set up commit/PR linting to (only) enforce PR titles, per squash merging

[JoshuaKGoldberg]

Tooling Suggestion Checklist

• I have tried restarting my IDE and the issue persists.
• I have pulled the latest master 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

Following #5038 (comment):

We're discussing this now. Our consensus is:

1. We don't believe the benefits of a pristine Git history to be worth the cost of asking everyone to maintain that history
2. Squash merging PRs using their title + description preserves a relatively clean history
3. We'll switch to squash merging always, and only enforcing conventions on the PR title
4. We can therefore remove commit linting for anything other than file formatting

Filing this issue to track setting up new / updating any existing workflows to match this preference.

Additional Info

No response

[JoshuaKGoldberg]

#5077 started on this.

[JoshuaKGoldberg]

I believe that was all we needed, actually! Closing out. 🙂