docs: add CLAUDE.md for Claude Code guidance (#248)

Add documentation file to provide context about codebase architecture, development commands, and key behaviors for Claude Code instances working in this repository.

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

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: kentaro-m

CLAUDE.md

@@ -0,0 +1,92 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+Auto Assign Action is a GitHub Action that automatically adds reviewers and/or assignees to pull requests when they are opened. The action reads a YAML configuration file (default: `.github/auto_assign.yml`) and assigns users based on single or grouped reviewer lists, with support for filtering by labels, draft status, and skip keywords.
+
+## Development Commands
+
+### Build and Test
+```bash
+npm run build          # Compile TypeScript to lib/ directory
+npm test               # Run all Jest tests
+npm run format         # Format all TypeScript files with Prettier
+npm run format-check   # Check formatting without modifying files
+npm run package        # Bundle with ncc into dist/index.js for distribution
+```
+
+### Running Individual Tests
+```bash
+npm test -- __tests__/utils.test.ts           # Run specific test file
+npm test -- -t "test name pattern"            # Run tests matching pattern
+```
+
+## Architecture
+
+### Entry Point Flow
+1. **main.ts** - Entry point that calls `run()`
+2. **run.ts** - Orchestrates the action:
+   - Fetches inputs (`repo-token`, `configuration-path`)
+   - Creates GitHub Octokit client
+   - Loads config from repository using `utils.fetchConfigurationFile()`
+   - Delegates to `handler.handlePullRequest()`
+
+### Core Components
+
+**handler.ts** - Main business logic
+- `handlePullRequest()`: Validates PR against filters (skip keywords, draft status, labels), then adds reviewers and assignees
+- `Config` interface: Defines all configuration options from YAML file
+- Filters execute in order: skip keywords → draft check → label filters → assignment
+
+**utils.ts** - Selection logic
+- `chooseReviewers()` / `chooseAssignees()`: Select users from single list or groups
+- `chooseUsers()`: Filters out PR creator, returns all users if `numberOfReviewers: 0`, otherwise randomly samples
+- `chooseUsersFromGroups()`: Iterates groups and selects users from each
+- `fetchConfigurationFile()`: Retrieves and parses YAML config from repository via GitHub API
+
+**pull_request.ts** - GitHub API wrapper
+- `PullRequest` class encapsulates context and client
+- `addReviewers()`: Calls `pulls.requestReviewers` API
+- `addAssignees()`: Calls `issues.addAssignees` API (PRs are issues)
+- `hasAnyLabel()`: Checks if PR has any of the specified labels
+
+### Configuration System
+
+The action expects a YAML configuration file (default `.github/auto_assign.yml`) with these key options:
+- `addReviewers` / `addAssignees`: Enable reviewer/assignee assignment
+- `reviewers` / `assignees`: Single list mode
+- `useReviewGroups` / `useAssigneeGroups`: Enable grouped mode
+- `reviewGroups` / `assigneeGroups`: Multiple lists keyed by group name
+- `numberOfReviewers` / `numberOfAssignees`: How many to assign (0 = all)
+- `skipKeywords`: Skip assignment if PR title contains these
+- `filterLabels.include` / `filterLabels.exclude`: Label-based filtering
+- `runOnDraft`: Whether to run on draft PRs (default: false)
+- Special: `addAssignees: "author"` assigns PR creator as assignee
+
+### Key Behaviors
+
+- **PR Creator Exclusion**: The PR author is always filtered out from reviewers/assignees (unless `addAssignees: "author"`)
+- **Random Sampling**: When `numberOfReviewers` > 0, uses `lodash.sampleSize()` for random selection
+- **Group Mode**: When using groups, the specified number is selected from EACH group, not total
+- **Error Handling**: Reviewer/assignee addition failures are logged as warnings, not failures
+- **Configuration Validation**: Throws errors if group flags are enabled but group lists are missing
+
+### Distribution
+
+The action uses `@vercel/ncc` to bundle everything into `dist/index.js`. After making changes:
+1. Run `npm run build` to compile TypeScript
+2. Run `npm run package` to create the bundled distribution
+3. Commit both `lib/` and `dist/` directories
+
+The action runs on Node 20 (specified in `action.yml`).
+
+### Testing
+
+Tests use Jest with ts-jest transformer. Test files mirror source structure:
+- `__tests__/utils.test.ts` - Tests selection logic and utility functions
+- `__tests__/handler.test.ts` - Tests main handler logic and filtering
+- `__tests__/run.test.ts` - Tests action entry point
+
+Mock `@actions/github` for testing without real API calls.