Make the confirmation message optional in Dangermattic's issue label validation

We currently have this Dangermattic confirmation when validating GitHub issues:

Adding a confirmation message (e.g. always adding a message) was simpler given some limitations on the `gh` CLI tool back when we first implemented the validation, but we should improve the code to make this confirmation message optional.

The issue names no files or tests. Start by locating Dangermattic's GitHub issue label validation entry point and tracing how the confirmation message is added; done means callers can choose whether to include the message while label validation continues to work.

Written by the indexing model from the issue text.