docs: improve S2 agent skill

[reidbarber]

This PR does the following:

1. Adds additional content to the S2 agent skill's SKILL.md for common mistakes observed when agents use the current skill. It is important that these are surfaced up to SKILL.md instead of being only in the reference files.
2. Adds a "Component Decision Tree" reference file that agents can consult when requirements don't mention a specific component by name.

✅ 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:

Install the skill:

npx skills add https://d1pzu54gtk2aed.cloudfront.net/pr/41efdfb5f8076ba700d1fc31482e18e6db8d483f/ --skill react-spectrum-s2

and invoke it when upgrading your project.

🧢 Your Project:

[rspbot]

Build successful! 🎉

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

[rspbot]

Build successful! 🎉

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