chore: add a Claude.md file to help with contributions

[snowystinger]

Closes

I mostly had Claude write this file. I dictated that it specifically include the guidelines around writing tests and the repo organisation. The rest and the file's structure are Claude with a little of my editing.

The guidelines around testing are ones that I've found useful in my own work to try to get better test suites out of Claude. Hopefully this will improve contribution quality as well.

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

🧢 Your Project:

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the S2 docs
• View the React Aria docs