docs: add CLAUDE.md

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.
+
+## Project Overview
+
+This is a GitHub Action that installs and configures mise, a polyglot runtime manager. The action is written in TypeScript and published to the GitHub Actions marketplace.
+
+## Development Commands
+
+```bash
+# Install dependencies
+npm install
+
+# Build, format, lint, and package
+npm run all
+
+# Individual commands
+npm run format:write  # Format code with Prettier
+npm run lint         # Run ESLint and format check
+npm run package      # Bundle with ncc for distribution
+
+# Testing
+npm run all          # Run full build pipeline
+./scripts/test.sh    # Integration test script
+```
+
+## Architecture
+
+The action follows GitHub's standard TypeScript action structure:
+
+1. **Entry Point**: `src/index.ts` - Main action logic that:
+   - Downloads and installs mise binary
+   - Manages caching through GitHub Actions cache
+   - Configures environment variables (MISE_*, GITHUB_TOKEN)
+   - Runs mise commands (install, reshim, etc.)
+   - Exports mise environment variables to GITHUB_ENV
+
+2. **Distribution**: `dist/index.js` - Compiled and bundled output (must be committed)
+
+3. **Action Definition**: `action.yml` - Defines inputs, outputs, and metadata
+
+## Key Implementation Details
+
+- **Cache Management**: Uses content-addressable caching based on mise config files (.mise.toml, .tool-versions, etc.)
+- **Binary Download**: Supports downloading from GitHub releases or mise.jdx.dev
+- **Platform Support**: Handles Linux (glibc/musl), macOS, and Windows
+- **Environment Setup**: Automatically adds mise bin and shims directories to PATH
+- **GitHub API**: Uses GITHUB_TOKEN to avoid rate limits when installing GitHub-hosted tools
+
+## Important Notes
+
+- Always run `npm run all` before committing to ensure dist/ is updated
+- The dist/ folder must be committed as GitHub Actions runs the compiled code
+- Test changes using the action itself (uses: ./) in test workflows