chore(repo): add CLAUDE.md for AI-assisted development

CLAUDE.md

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+# Clerk JavaScript SDK Monorepo
+
+## Workflow
+- Use Plan mode for cross-package changes or new features
+- Run `pnpm build --filter=@clerk/[package]...` to build a package with dependencies
+- Changes to `@clerk/shared` or `@clerk/types` require rebuilding dependent packages
+
+## Verification
+Before marking work complete:
+- `pnpm build` passes (or filtered build for touched packages)
+- `pnpm test` passes
+- `pnpm lint` passes
+- Changeset added if user-facing change: `pnpm changeset`
+
+## Project Structure
+```
+packages/           # 31 SDK packages
+  clerk-js/         # Core JS (UI components, auth flows)
+  react/            # React SDK (hooks, components)
+  nextjs/           # Next.js SDK
+  backend/          # Node.js backend SDK
+  shared/           # Shared utilities (used by most packages)
+  types/            # Shared TypeScript types
+  express|fastify|  # Framework integrations
+  vue|astro|expo/   # More framework SDKs
+integration/        # E2E tests (Playwright)
+playground/         # Demo apps for testing
+```
+
+## Key Technologies
+pnpm 10+ | Turbo | TypeScript 5.8 | tsup | Vitest | Playwright | ESLint 9 (flat config)
+
+## Code Patterns
+
+**Package exports:**
+- `.` - Public API
+- `./internal` - Internal APIs (prefix: `__internal_`)
+- `./experimental` - Experimental APIs (prefix: `__experimental_`)
+- `./errors` - Error classes
+
+**Naming conventions:**
+- Hooks: `use[Feature]` (e.g., `useUser`, `useAuth`)
+- Components: PascalCase (e.g., `SignIn`, `UserButton`)
+- Internal: `__internal_[name]`
+- Experimental: `__experimental_[name]`
+
+**Build output:** Dual ESM (.mjs) + CJS (.js) with .d.ts declarations
+
+**Commit format:** `feat(package-name): description` or `fix(package-name): description`
+
+## Common Commands
+| Command | Description |
+|---------|-------------|
+| `pnpm build` | Build all packages |
+| `pnpm build --filter=@clerk/react...` | Build package + deps |
+| `pnpm dev` | Watch mode for development |
+| `pnpm test` | Run all unit tests |
+| `pnpm test --filter=@clerk/react` | Test single package |
+| `pnpm lint` | ESLint check |
+| `pnpm format` | Prettier format |
+| `pnpm changeset` | Add changeset for release |
+
+## Testing
+- **Unit tests:** Vitest (co-located in `__tests__/` or `.test.ts` files)
+- **E2E tests:** `pnpm test:integration:base` (Playwright)
+- **Test single file:** `pnpm vitest run path/to/file.test.ts`
+
+## Learnings
+<!-- Add gotchas with dates, prune when stale -->
+<!-- 2025-01-30: Example - clerk-js requires specific build order -->