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/components/thread-pane.test.tsx

@@ -0,0 +1,36 @@
+import { describe, it, expect } from 'vitest';
+import { render, screen, fireEvent, waitFor } from '@testing-library/react';
+import { MessagingProvider } from '../provider';
+import { Messenger } from './messenger';
+import { MockAdapter } from '../adapters/mock';
+
+function mount() {
+  return render(
+    <MessagingProvider adapter={new MockAdapter()}>
+      <Messenger />
+    </MessagingProvider>,
+  );
+}
+
+describe('Slack-style thread pane', () => {
+  it('opens a thread on 💬, posts a reply into it, and shows the reply count on the parent', async () => {
+    mount();
+    // Wait for the auto-selected thread's message to render WITH its actions (not just the sidebar
+    // preview), then open the thread pane via the message's reply affordance (💬).
+    const replyButtons = await screen.findAllByTitle('Reply in thread');
+    fireEvent.click(replyButtons[0]!);
+    expect(await screen.findByText('Thread')).toBeTruthy(); // pane header
+
+    // The pane's composer (placeholder "Reply…") — send a threaded reply.
+    const replyInput = screen.getByPlaceholderText('Reply…') as HTMLInputElement;
+    fireEvent.change(replyInput, { target: { value: 'on it' } });
+    // The pane has its own Send; grab the last one (pane is rendered after the main composer).
+    const sends = screen.getAllByText('Send');
+    fireEvent.click(sends[sends.length - 1]!);
+
+    // The reply shows in the pane (replies are hidden from the main thread, so this is unique)...
+    await waitFor(() => expect(screen.getByText('on it')).toBeTruthy());
+    // ...and the parent now advertises the reply count as a thread-link button in the main thread.
+    await waitFor(() => expect(screen.getByRole('button', { name: /1 reply/ })).toBeTruthy());
+  });
+});