Add Simple Retry Mechanism for Rate Limits

[jordanpartridge]

Implement basic retry logic for rate limit errors:

// In config/github-client.php
'retry' => [
    'max_attempts' => 3,
    'delay' => 1000, // milliseconds
    'only_on' => [
        GithubRateLimitException::class,
    ],
],

Implementation:

1. Add retry config options
2. Create RetryMiddleware
3. Add to connector pipeline
4. Add tests for retry behavior

This provides automatic handling of temporary rate limit issues.