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/kernel-client.ts

@@ -133,11 +133,11 @@ export class KernelClientAdapter implements MessagingAdapter {
     // Attachments are intentionally not forwarded here: kernel-client exposes no media/presign yet,
     // and the SDK Attachment carries a display `url`, not a storage `contentRef`. Media is a follow-up
     // (kernel-client media methods + a contentRef on Attachment). Text + reply threading work today.
-    const m = await this.socket.sendMessage(
-      threadId,
-      content,
-      opts?.parentInteractionId ? { parentInteractionId: opts.parentInteractionId } : undefined,
-    );
+    const sendOpts = {
+      ...(opts?.parentInteractionId ? { parentInteractionId: opts.parentInteractionId } : {}),
+      ...(opts?.mentions && opts.mentions.length ? { mentions: opts.mentions } : {}),
+    };
+    const m = await this.socket.sendMessage(threadId, content, Object.keys(sendOpts).length ? sendOpts : undefined);
     return this.toMessage(m);
   }
 

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++}`;