feat(messaging-ui): start direct messages + groups (Slack-style people picker)

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>

packages/iios-messaging-ui/src/adapter.ts

@@ -61,6 +61,10 @@ export interface MessagingAdapter {
    *  Absent => the composer offers no autocomplete (you can still type @text). */
   listMembers?(threadId: string): Promise<Person[]>;
 
+  /** The people you can start a conversation with (org directory). Drives the "New message"
+   *  people picker. Absent => the UI hides DM/group creation (you can still open channels). */
+  directory?(): Promise<Person[]>;
+
   // ── Channels (optional capability) ──────────────────────────────
   // A channel is just a third membership beyond dm/group: a discoverable, joinable room.
   // Implement all four to enable the channels UI; absent => the UI hides channels entirely.