Add CLAUDE.md with SDK development guide

CLAUDE.md

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+# MCP TypeScript SDK Guide
+
+## Build & Test Commands
+```
+npm run build        # Build ESM and CJS versions
+npm run lint         # Run ESLint
+npm test             # Run all tests
+npx jest path/to/file.test.ts  # Run specific test file
+npx jest -t "test name"        # Run tests matching pattern
+```
+
+## Code Style Guidelines
+- **TypeScript**: Strict type checking, ES modules, explicit return types
+- **Naming**: PascalCase for classes/types, camelCase for functions/variables
+- **Files**: Lowercase with hyphens, test files with `.test.ts` suffix
+- **Imports**: ES module style, include `.js` extension, group imports logically
+- **Error Handling**: Use TypeScript's strict mode, explicit error checking in tests
+- **Formatting**: 2-space indentation, semicolons required, single quotes preferred
+- **Testing**: Co-locate tests with source files, use descriptive test names
+- **Comments**: JSDoc for public APIs, inline comments for complex logic
+
+## Project Structure
+- `/src`: Source code with client, server, and shared modules
+- Tests alongside source files with `.test.ts` suffix
+- Node.js >= 18 required