🛠 Tooling: Add initial Copilot instructions

[JoshuaKGoldberg]

Tooling Report Checklist

• I have tried restarting my IDE and the issue persists.
• I have pulled the latest main branch of the repository.
• I have searched for related issues and found none that matched my issue.

Overview

Related to #354: there are some persistent settings that AI agents such as Copilot should know. I'll add a persistent file for them that can be kept updated.

Additional Info

I'm still actively changing the notes from #354 as Copilot PR commits come in. I'll wait until they're a little more settled before moving those notes to a file.

❤️‍🔥

[JoshuaKGoldberg]

I've mostly settled down with them. Plus, I think Copilot is forgetting PR context if I remove it from pull request descriptions 😬. Rephrasing this as "initial" instructions and un-blocking.