Add --label flag to search custom label names

Problem: The CLI hardcodes "Good First Issue" as the search label, so users cant search for other helpful labels like "help wanted" or "first-timers-only".

Solution: Add a `--label` (or `-l`) flag that lets users specify a custom label name to search for. Default remains "Good First Issue" for backward compatibility.

Use case: Contributors looking for issues labeled "help wanted" or "good-first-issue" in other languages/localizations.

Example: `good-first-issue --label help-wanted` or `good-first-issue -l enhancement`

This is a small, well-scoped CLI enhancement with a clear user benefit.

Start at the CLI entry point and find where the hard-coded "Good First Issue" label is used for searches. Add support for --label and -l while preserving that default, then verify the examples and existing behavior from the command line.

Written by the indexing model from the issue text.