Add .github/copilot-instructions.md for AI agent onboarding

[Copilot]

Creates comprehensive onboarding documentation for AI coding agents working with this React component library.

File Structure

.github/copilot-instructions.md (481 lines):

• Project context: TypeScript-first React 18 component library with server/client component architecture
• Tech stack: React 19, TypeScript 5.7, Tailwind CSS 3.4, tsup/tsc build, Vitest testing, Storybook docs
• Critical constraints:
  • Must use pnpm 10.9.0 (not npm/yarn)
  • Storybook runs in Docker (don't start locally)
  • Server actions must separate from client components
• Build process: Dual tsup (bundling) + tsc (types) with directive preservation
• Known issues documented:
  • TypeScript errors in example files (excluded from build, ignorable)
  • 11 pre-existing test failures (don't fix unless related)
  • pnpm installation requirement
  • Server/client directive conflicts with solutions
• Development patterns: Component creation workflow, export chain, testing with @testing-library/react
• Architecture decisions: Why two CSS files (styles.css vs globals.css), separate server exports, tsup+tsc rationale

Key Sections

• Package manager requirements and installation
• Essential commands: pnpm run build && pnpm run typecheck && pnpm run test
• Project structure with path aliases
• Styling system (Tailwind preset for consumers)
• Common pitfalls with solutions
• Quick reference commands
• AI agent checklist

All errors encountered during exploration are documented with workarounds per the requirements.

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.