add CLAUDE.md (#75)

Author: uhyo

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Architecture Overview
+
+This project generates alternative TypeScript standard library definitions with improved type safety. The architecture consists of:
+
+- **`build/`** - TypeScript build system and generation logic
+  - `lib.ts` - Main script for generating improved lib definitions
+  - `package.ts` - Script for building npm packages
+  - `diff.ts` - Script for generating documentation diffs
+  - `logic/` - Core generation logic and AST manipulation
+
+- **`lib/`** - Source TypeScript lib definition improvements (what gets applied to TypeScript's built-in libs)
+
+- **`generated/`** - Output directory for processed TypeScript lib files
+
+- **`dist-package/`** - Individual npm packages for each TypeScript lib (e.g., es2015, dom, etc.)
+
+- **`TypeScript/`** - Git submodule containing the official TypeScript repository for source lib files
+
+- **`tests/`** - Type-level tests using `tsd` to verify the improved type definitions work correctly
+
+## Essential Commands
+
+### Build Commands
+```bash
+# Compile TypeScript build scripts
+npm run build:tsc
+
+# Generate improved lib files from TypeScript source + better-typescript-lib improvements
+npm run build:lib
+
+# Build npm packages for distribution
+npm run build:package
+
+# Generate documentation diffs
+npm run build:diff
+```
+
+### Testing
+```bash
+# Run type-level tests in tests/ directory
+cd tests && npm test
+
+# Or run tsd directly
+cd tests && npx tsd
+```
+
+### Development Workflow
+1. Modify type definitions in `lib/` directory
+2. Run `npm run build:tsc && npm run build:lib` to regenerate
+3. Test changes with `cd tests && npm test`
+4. Generate packages with `npm run build:package` if needed
+
+## Key Implementation Details
+
+- Uses AST manipulation to apply improvements to TypeScript's standard library definitions
+- Generates separate npm packages that TypeScript automatically detects and uses (via `@typescript/lib-*` naming)
+- The `ReplacementMap` system tracks which parts of the original TypeScript libs should be replaced with improved versions
+- Type improvements focus on replacing `any` with more specific types like `JSONData` for `JSON.parse()`
+- All changes prioritize type safety over convenience, potentially causing breaking changes in existing codebases
+
+## Important Files
+- `build/logic/generate.ts` - Core generation algorithm
+- `build/logic/ReplacementMap.ts` - Tracks lib modifications  
+- `lib/es5.d.ts` - Main improvements to core JavaScript APIs
+- `tests/src/` - Type-level tests for verifications