updates the ui-agent instructions to provide clearer expectations

[jamespepper81]

This pull request updates the ui-agent instructions to provide clearer expectations and more actionable guidance for reviewing UI/UX in PRs. The changes expand on the agent's responsibilities, clarify the tone and structure of responses, and add more diverse example prompts.

Updates to agent responsibilities and guidance:

• Expanded the list of responsibilities to explicitly include reviewing PRs for UI/UX quality and recommending modern React Native design patterns.
• Added guidance to ensure responses are constructive, actionable, and concise, and to provide reasoning and improved code examples when relevant.

Enhancements to example prompts:

• Added new example requests covering performance optimization, modern mobile UI design, cross-platform considerations, and animation recommendations.