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);
   }