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/index.ts

@@ -7,6 +7,7 @@ export { MessagingProvider, useAdapter } from './provider';
 export { useConversations } from './hooks/use-conversations';
 export { useMessages } from './hooks/use-messages';
 export { useChannels } from './hooks/use-channels';
+export { useMembers } from './hooks/use-members';
 export { isOwnMessage } from './types';
 
 // Rendered UI. Pair with the './styles.css' export (or override the --miu-* tokens).