Housekeeping subtypes (channel_join/leave/topic/name/purpose,
convert_to_private/public, pins, deletions) are not a person speaking,
yet _prefilter_inbound only rejected message_changed/message_deleted,
so each of them started a full agent turn in free-response channels.
Replace the denylist with an allowlist: a message passes when subtype
is absent, file_share, thread_broadcast or me_message; everything else
is dropped. Fixes#110778.