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/hooks/use-members.ts

@@ -0,0 +1,31 @@
+import { useEffect, useState } from 'react';
+import { useAdapter } from '../provider';
+import type { Person } from '../types';
+
+/** A thread's members (for @mention autocomplete + highlighting). Empty if the adapter
+ *  doesn't implement listMembers, or while loading. */
+export function useMembers(threadId: string | null): Person[] {
+  const adapter = useAdapter();
+  const [members, setMembers] = useState<Person[]>([]);
+
+  useEffect(() => {
+    if (!threadId || !adapter.listMembers) {
+      setMembers([]);
+      return;
+    }
+    let alive = true;
+    adapter
+      .listMembers(threadId)
+      .then((m) => {
+        if (alive) setMembers(m);
+      })
+      .catch(() => {
+        if (alive) setMembers([]);
+      });
+    return () => {
+      alive = false;
+    };
+  }, [adapter, threadId]);
+
+  return members;
+}