docs: add workspace copilot instructions

[austenstone]

Before the change?

• The repository did not have workspace-level Copilot instructions capturing maintainer review patterns and repo-specific contribution expectations.

After the change?

• Add .github/copilot-instructions.md with concise project guidance based on recent merged PR review history and existing contribution docs.
• Document the review patterns maintainers consistently enforce around scope, SDK patterns, tests, docs, and validation.

Pull request checklist

• Schema migrations have been created if needed
• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

• Yes
• No

[github-actions]

👋 Hi! Thank you for this contribution! Just to let you know, our GitHub SDK team does a round of issue and PR reviews twice a week, every Monday and Friday! We have a process in place for prioritizing and responding to your input. Because you are a part of this community please feel free to comment, add to, or pick up any issues/PRs that are labeled with Status: Up for grabs. You & others like you are the reason all of this works! So thank you & happy coding! 🚀

[austenstone]

Friendly bump — any chance this could get a review? Happy to address feedback. Thanks!