Purges in private channels get logged to mod-log

I'm not sure how you would like to handle this, but there should be some way to use message purging without it leaking from private channels to public.

The issue names no files, tests, or entry points; start by locating message-purge handling and the mod-log output path in the Java bot. Confirm the intended privacy boundary with maintainers, then verify that purges in private channels no longer appear in public moderation logs while public-channel logging remains intact.

Written by the indexing model from the issue text.