Add a 'New message' + on the Direct messages section that opens a people picker
(NewConversation): pick one person -> DM, two or more -> group with an optional
name, via the adapter's existing openThread({participantIds, membership, subject}).
Expose directory() on MessagingAdapter (org people list); MockAdapter implements
it + dedupes 1:1 DMs. 72 tests green.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
