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/adapters/mock.ts

@@ -159,7 +159,20 @@ export class MockAdapter implements MessagingAdapter {
     });
   }
 
+  async directory(): Promise<Person[]> {
+    return MOCK_PEOPLE.map((p) => ({ ...p }));
+  }
+
   async openThread(p: { participantIds: string[]; membership?: Membership; subject?: string }): Promise<{ threadId: string }> {
+    // A DM to someone you already have reuses the existing 1:1 thread (dedupe, like the live door).
+    if ((p.membership ?? (p.participantIds.length === 1 ? 'dm' : 'group')) === 'dm' && p.participantIds.length === 1) {
+      const target = p.participantIds[0];
+      for (const [id, t] of this.threads) {
+        if (t.membership === 'dm' && t.participants.length === 2 && t.participants.includes(ME) && t.participants.includes(target)) {
+          return { threadId: id };
+        }
+      }
+    }
     const membership = p.membership ?? (p.participantIds.length === 1 ? 'dm' : 'group');
     const threadId = `th_mock_${this.seq++}`;
     this.threads.set(threadId, {