feat(messaging-ui): @mentions — autocomplete, resolve to ids, highlight

- contract: SendOpts.mentions (opaque userId notify-list) + optional
  listMembers(threadId) for autocomplete/highlight (capability-degrading)
- mock: listMembers; KernelClientAdapter.send forwards mentions to the socket
  (which already carries them → inbox MENTION items)
- composer: type @ → member/@channel/@here autocomplete; Enter picks the first;
  on send, mentions resolve to ids; message bodies highlight @mentions
- useMembers hook; mentions.tsx helpers (trailingMentionQuery/insertMention/
  resolveMentions/highlightMentions)
- 4 mention tests (helpers + render autocomplete→send carries id); 56 total green

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/iios-messaging-ui/src/adapters/mock.ts

@@ -150,6 +150,15 @@ export class MockAdapter implements MessagingAdapter {
     if (t) t.participants = t.participants.filter((p) => p !== ME);
   }
 
+  async listMembers(threadId: string): Promise<Person[]> {
+    const t = this.threads.get(threadId);
+    if (!t) return [];
+    return t.participants.map((id) => {
+      if (id === ME) return { id: ME, name: 'You', kind: 'staff' };
+      return MOCK_PEOPLE.find((p) => p.id === id) ?? { id, name: id, kind: 'staff' };
+    });
+  }
+
   async openThread(p: { participantIds: string[]; membership?: Membership; subject?: string }): Promise<{ threadId: string }> {
     const membership = p.membership ?? (p.participantIds.length === 1 ? 'dm' : 'group');
     const threadId = `th_mock_${this.seq++}`;