feat(08-02): integrate message list into chat window

Replace placeholder content with JKChatMessageList component:
- Import JKChatMessageList
- Remove placeholder div with text
- JKChatMessageList handles its own styling (flex: 1, padding, scroll)
- Update component documentation

Chat window now displays:
- Header with channel name and close button (fixed top)
- Message list with auto-scroll (scrollable, flex: 1)
- Loading spinner when fetching history
- Empty state when no messages
- Individual messages with avatars and timestamps

Layout follows flexbox pattern with header fixed and message list
filling remaining space with independent scrolling.

Ready for Plan 8.3 (Chat Button & Unread Badge).

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

jam-ui/src/components/client/JKSessionChatWindow.js

@@ -2,6 +2,7 @@ import React, { useCallback } from 'react';
 import { useSelector, useDispatch } from 'react-redux';
 import WindowPortal from '../common/WindowPortal.js';
 import JKChatHeader from './chat/JKChatHeader.js';
+import JKChatMessageList from './chat/JKChatMessageList.js';
 import {
   selectIsChatWindowOpen,
   selectActiveChannel,
@@ -20,9 +21,9 @@ import {
  * - closeChatWindow: Action to close window
  *
  * Component hierarchy:
- * WindowPortal → JKChatHeader + [content area placeholder]
+ * WindowPortal → JKChatHeader + JKChatMessageList
  *
- * Note: Message list implementation deferred to Plan 8.2
+ * Note: Message composition implementation deferred to Plan 9
  */
 const JKSessionChatWindow = () => {
   const dispatch = useDispatch();
@@ -60,12 +61,7 @@ const JKSessionChatWindow = () => {
           channelName={getChannelName()}
           onClose={handleClose}
         />
-        <div style={{ flex: 1, padding: '16px', backgroundColor: '#ffffff' }}>
-          {/* Message list placeholder - Plan 8.2 */}
-          <p style={{ color: '#6c757d', textAlign: 'center', marginTop: '24px' }}>
-            Chat messages will appear here
-          </p>
-        </div>
+        <JKChatMessageList />
       </div>
     </WindowPortal>
   );