chore: init CLAUDE.md (#9563)

* chore: init CLAUDE.md

* Add Copilot instructions

* update gitignore

* simplify

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Co-authored-by: Mark Tolmacs <mark@lazycat.hu>

CLAUDE.md

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+# CLAUDE.md
+
+## Project Structure
+
+Excalidraw is a **monorepo** with a clear separation between the core library and the application:
+
+- **`packages/excalidraw/`** - Main React component library published to npm as `@excalidraw/excalidraw`
+- **`excalidraw-app/`** - Full-featured web application (excalidraw.com) that uses the library
+- **`packages/`** - Core packages: `@excalidraw/common`, `@excalidraw/element`, `@excalidraw/math`, `@excalidraw/utils`
+- **`examples/`** - Integration examples (NextJS, browser script)
+
+## Development Workflow
+
+1. **Package Development**: Work in `packages/*` for editor features
+2. **App Development**: Work in `excalidraw-app/` for app-specific features
+3. **Testing**: Always run `yarn test:update` before committing
+4. **Type Safety**: Use `yarn test:typecheck` to verify TypeScript
+
+## Development Commands
+
+```bash
+yarn test:typecheck  # TypeScript type checking
+yarn test:update     # Run all tests (with snapshot updates)
+yarn fix             # Auto-fix formatting and linting issues
+```
+
+## Architecture Notes
+
+### Package System
+
+- Uses Yarn workspaces for monorepo management
+- Internal packages use path aliases (see `vitest.config.mts`)
+- Build system uses esbuild for packages, Vite for the app
+- TypeScript throughout with strict configuration