Make CodeBunny installable via a GitHub App Marketplace listing

## Problem

Currently, users need to manually create and configure their own GitHub App to use CodeBunny with enhanced permissions. This adds friction to the installation process and requires users to understand GitHub App setup, permissions, and configuration.

## Proposed Solution

Create an official CodeBunny GitHub App that users can install directly from the GitHub Marketplace or via a simple installation link. This would streamline the onboarding process significantly.

## Benefits

- **One-click installation** - Users can install the app directly without manual GitHub App creation
- **Pre-configured permissions** - App comes with the correct permissions already set (Contents: Read, Issues: Write, Pull Requests: Write)
- **Simplified setup** - Reduces installation steps and potential configuration errors
- **Better user experience** - Aligns with how users expect to install GitHub Actions/integrations
- **Discoverability** - GitHub Marketplace listing would help more users find CodeBunny

## Implementation Details

1. Create an official GitHub App for CodeBunny
2. Set up the required permissions:
- Contents: Read
- Issues: Write
- Pull Requests: Write
3. Configure webhook events for pull requests and issue comments
4. Create a GitHub App Marketplace listing
5. Update documentation to include the simple installation flow
6. Maintain backward compatibility with the current manual setup

## User Flow (After Implementation)

1. Visit GitHub App installation page or Marketplace listing
2. Click "Install" and select repositories
3. Add Continue API key and configuration to repository secrets
4. Add workflow file (simplified, no APP_ID or APP_PRIVATE_KEY needed)
5. Done!

## Related Documentation

- Current setup requires users to follow: [Detailed GitHub App Setup Guide](actions/codebunny/README.md#prerequisites)
- This enhancement would make Step 1 of installation obsolete for most users

## Open Questions

- Should we maintain support for custom GitHub Apps alongside the official app?
- What naming should we use for the official app?
- Should the app be organization-wide or per-repository by default?