find-inactive-members.rb missing GitHub App option

For large enterprise GitHub environment, Personal Access Token will be insufficient due to API rate limiting. Please update `find-inactive-members.rb` with a GitHub App option.

Start with find-inactive-members.rb and inspect how the current Personal Access Token authentication and GitHub API calls are organized. Define the GitHub App option in that script and verify that the inactive-member lookup works with it alongside the existing authentication path.

Written by the indexing model from the issue text.