Bug: Syncing labels deletes *all* existing labels in a repo (and then repopulates)

This removes all the labels currently assigned to issues, which reduces the usefulness of the tool.

Desired functionality.

1. Tool should de-dupe and only add labels to a repo if those labels don't already exist
2. Tool should only delete labels that are not part of the master list (this should be optional)

The issue names no files, entry points, or tests. Locate the label-sync logic, reproduce the deletion and repopulation behavior, then verify that existing labels are preserved, duplicates are avoided, and deletion of labels outside the master list is optional.

Written by the indexing model from the issue text.