Error handling

Authentication or network failures set the process exit code to a non-zero value and emit a clear error message describing the failing step (e.g. "GitHub rejected the token: Bad credentials").

Common error scenarios:

• Missing required arguments — Either --pr-url or both --owner and --repo must be provided.
• Authentication failure — Token is missing, empty, or rejected by GitHub.
• Rate limit exceeded — API rate limit reached; wait for reset time.
• Network errors — Cannot reach the GitHub API endpoint.