feat: add sanity-ui skill

[nkgentile]

Description

Adds a sanity-ui skill to the Sanity Agent Toolkit, providing AI agents with guidance for building React interfaces using Sanity's official design system (@sanity/ui).

Changes introduced:

• New skill at sanity-plugin/skills/sanity-ui/SKILL.md - Concise, procedure-focused workflow guide for building UIs with Sanity UI
• New rule at rules/sanity-ui.mdc - Comprehensive reference covering design system fundamentals, component selection, and Sanity-specific patterns
• Updated AGENTS.md knowledge router to include Sanity UI triggers
• Updated README.md to document the new skill and rule

Why these changes?

• Enables agents to help developers build custom Studio plugins, tools, and Studio UI components using Sanity's design system
• Provides clear guidance on when ThemeProvider is needed (standalone apps) vs. auto-provided (Studio context)
• Follows established repository patterns: concise skills for workflows, comprehensive rules for reference
• Avoids duplicating official API docs (links to https://www.sanity.io/ui instead), focusing on Sanity-specific integration patterns

What to review

Reviewer should:

1. Review sanity-plugin/skills/sanity-ui/SKILL.md:

   • Verify the 5-step procedure is clear and actionable
   • Confirm ThemeProvider guidance is accurate (Studio vs standalone context)
   • Check that examples demonstrate both Studio plugin and standalone app patterns

2. Review rules/sanity-ui.mdc:

   • Verify design system fundamentals (spacing scale, tones, responsive patterns) are accurate
   • Confirm component quick reference is helpful without duplicating official docs
   • Check that Sanity-specific patterns (document panels, custom inputs, etc.) are correct
   • Ensure it points to official docs for detailed API reference

3. Review AGENTS.md and README.md:

   • Verify routing keywords are appropriate for triggering the skill
   • Confirm documentation accurately describes the skill's purpose

Affected areas:

• Claude Code plugin skills (adds new sanity-ui skill)
• Context rules (adds new sanity-ui.mdc rule)
• Knowledge routing (updates trigger keywords)

Testing

Manual testing performed:

• Verified file structure matches existing skill patterns (e.g., sanity-groq, sanity-scaffold)
• Confirmed line count is appropriate (~138 lines for skill, ~519 lines for rule)
• Reviewed ThemeProvider guidance against actual Studio codebase at ~/Developer/sanity
• Validated that skill references point correctly to rule file via sanity://rules/sanity-ui.mdc
• Confirmed examples use correct imports and API patterns from @sanity/ui