ci: add Claude code review GitHub Action

- Add automated code review workflow that triggers on PRs
- Support on-demand reviews via @claude mentions
- Configure review to check code quality, architecture, testing, and performance
- Add comprehensive setup documentation in docs/github-actions-setup.md
- Review focuses on project-specific conventions from CLAUDE.md

docs/github-actions-setup.md

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+# GitHub Actions Setup
+
+This document describes the GitHub Actions workflows configured for this repository.
+
+## Claude Code Review
+
+The repository is configured with automated code reviews powered by Claude AI.
+
+### How It Works
+
+The Claude Code Review action triggers:
+- **Automatically** on all pull requests (opened, synchronized, or reopened)
+- **On-demand** when you comment `@claude` on a pull request
+
+### Setup Requirements
+
+#### 1. Add Anthropic API Key
+
+You need to add your Anthropic API key as a repository secret:
+
+1. Go to your repository Settings
+2. Navigate to **Secrets and variables** → **Actions**
+3. Click **New repository secret**
+4. Name: `ANTHROPIC_API_KEY`
+5. Value: Your Anthropic API key (get one at https://console.anthropic.com)
+6. Click **Add secret**
+
+#### 2. Install Claude GitHub App (Optional)
+
+For the easiest setup, you can install the official Claude GitHub app:
+- Visit https://github.com/apps/claude
+- Click "Install"
+- Select your repository
+
+Alternatively, use the CLI: Open Claude Code and run `/install-github-app`
+
+### Review Focus Areas
+
+Claude reviews focus on:
+
+- **Code Quality**: Bugs, edge cases, error handling, security vulnerabilities
+- **Architecture**: Adherence to project conventions (CLAUDE.md), proper use of EventStore/RelayLiveness singletons
+- **Testing**: Coverage of parsers and pure functions
+- **Performance**: Unnecessary re-renders, subscription leaks, optimization opportunities
+
+### Permissions
+
+The workflow uses these permissions:
+- `contents: read` - Read repository code
+- `pull-requests: write` - Post review comments on PRs
+- `issues: write` - Respond to @claude mentions
+
+### Customizing Reviews
+
+To customize the review behavior, edit the `prompt` section in `.github/workflows/claude-code-review.yml`.
+
+### Resources
+
+- [Claude Code GitHub Actions Documentation](https://code.claude.com/docs/en/github-actions)
+- [How to Use Claude Code for PRs and Code Reviews](https://skywork.ai/blog/how-to-use-claude-code-for-prs-code-reviews-guide/)
+- [Integrating Claude Code with GitHub Actions](https://stevekinney.com/courses/ai-development/integrating-with-github-actions)