feat(messaging-ui): Slack-style threaded-replies side panel

Replies (messages with parentInteractionId) now open in a right-hand ThreadPane
instead of inline quoting:
- extracted a shared Composer (draft + @mention autocomplete + attach) and
  MessageItem (bubble + mentions + attachment + reactions + reply affordance)
- Thread shows top-level messages only; a message with replies gets a
  '💬 N replies' link, and hovering shows 💬 'Reply in thread'
- ThreadPane renders the root + its replies + a composer that posts back with
  parentInteractionId; Messenger becomes 3-column (list | thread | pane),
  pane closes on conversation switch
- no contract/adapter/backend change (parentInteractionId was already wired)
- thread-pane render test (open → reply → parent shows the count); 58 green

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

packages/iios-messaging-ui/src/index.ts

@@ -15,6 +15,7 @@ export { Messenger } from './components/messenger';
 export { ConversationList } from './components/conversation-list';
 export { ChannelBrowser } from './components/channel-browser';
 export { Thread } from './components/thread';
+export { ThreadPane } from './components/thread-pane';
 
 export type { MessagingAdapter } from './adapter';
 export type { ConversationsState } from './hooks/use-conversations';