🔌 Improve GitHub API error handling

[jordanpartridge]

Problem

All GitHub API failures return null silently, making it hard to distinguish between auth failures, rate limits, and transient errors.

Current Behavior

File: app/GitHub/ChecksClient.php:90-93

} catch (GuzzleException $e) {
    $this->logError('createCheck', $e);
    return null;
}

Expected Behavior

Differentiate between error types with specific messages.

Proposed Implementation

} catch (GuzzleException $e) {
    if ($e->getCode() === 401) {
        throw new \RuntimeException("GitHub token invalid or expired");
    }
    if ($e->getCode() === 403 && str_contains($e->getMessage(), 'rate limit')) {
        throw new \RuntimeException("GitHub API rate limit exceeded");
    }
    $this->logError('createCheck', $e);
    return null;
}

Benefits

• Better error messages for common issues
• Helps users fix auth problems quickly
• Distinguishes between retryable and permanent failures