How do I change the reply editor to use single line spacing by default for all agents?

There is no global admin setting for editor line spacing. Agents can set it individually in their Profile > Preferences > Editor Spacing. Users have no setting at all. Both agents and users can use Shift+Enter to insert a single-spaced newline regardless of the preference. To force single spacing globally, you must modify the codebase -- search GitHub for 'editor spacing' or 'tinyMCE' in the osTicket repository.