Add CLAUDE.md for Claude Code guidance

This file provides essential guidance for future Claude instances working
on this repository, including:

- Common development commands
- High-level architecture overview
- Testing structure and local testing instructions
- Build process details
- Key design decisions

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

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.
+
+## Overview
+
+This is a GitHub Action written in Node.js that detects breaking API changes in Go code using the official `golang.org/x/exp/cmd/apidiff` tool. It analyzes Go API compatibility between commits and can comment on pull requests with detailed change reports.
+
+## Common Commands
+
+### Development
+
+```bash
+# Install dependencies
+npm ci
+
+# Run tests
+npm test
+
+# Run linting
+npm run lint
+
+# Run formatter
+npm run format
+
+# Build the action (compiles to dist/)
+npm run build
+
+# Run pre-commit hooks manually
+pre-commit run --all-files
+```
+
+### Testing the Action Locally
+
+The action supports testing with environment variables:
+
+```bash
+# Test with directories
+INPUT_OLD=testdata/break/old INPUT_NEW=testdata/break/new node index.js
+
+# Test with git refs
+INPUT_OLD=HEAD~1 INPUT_NEW=HEAD node index.js
+```
+
+## Architecture
+
+### Core Components
+
+1. **index.js** - Main entry point that:
+
+   - Reads GitHub Action inputs
+   - Determines which commits/directories to compare
+   - Orchestrates the apidiff run, parsing, and PR commenting
+   - Sets action outputs
+
+2. **lib/apidiff.js** - Handles running the official apidiff tool:
+
+   - Installs `golang.org/x/exp/cmd/apidiff` if not present
+   - Supports both git refs and directory comparisons
+   - For directories: creates export files first, then compares them
+   - For git refs: runs apidiff directly on the commits
+
+3. **lib/parser.js** - Parses apidiff text output:
+
+   - Extracts breaking and compatible changes by package
+   - Converts to structured format
+   - Generates markdown reports for PR comments
+
+4. **lib/commenter.js** - Manages PR comments:
+   - Creates or updates existing comments (identified by hidden tag)
+   - Requires `pull-requests: write` permission
+   - Uses GitHub Actions toolkit for API access
+
+### Testing Structure
+
+The `testdata/` directory contains test cases:
+
+- `clean/` - No API changes
+- `safe/` - Only compatible changes (additions)
+- `break/` - Contains breaking changes
+
+Each test case has `old/` and `new/` subdirectories with Go code.
+
+### Build Process
+
+The action uses `@vercel/ncc` to compile all dependencies into a single `dist/index.js` file. This is required for GitHub Actions and must be committed to the repository.
+
+### Pre-commit Hooks
+
+Configured with husky to run:
+
+- ESLint and Prettier on JS files
+- Pretty-format-json on JSON files (excluding package-lock.json)
+- Builds dist/ and verifies it's up to date
+
+## Key Design Decisions
+
+1. **Node.js wrapper around Go tool**: Rather than reimplementing apidiff, this action wraps the official tool
+2. **Support for both directories and git refs**: Allows testing without git commits
+3. **Structured parsing**: Converts free-text apidiff output into structured data for better formatting
+4. **Comment management**: Updates existing comments rather than creating duplicates
+
+## Workflow Guidance
+
+- When you open a PR, enable auto-merge, and watch CI checks until they complete and the PR merges. If CI fails, use the gh CLI to diagnose the error