- Add scrollToMessageId/setScrollToMessageId to ChatPanelContext
- Add "Search chat messages" item to CommandPalette (dispatches nexus:open-chat-search)
- Integrate ChatSearchDialog, ChatBranchSelector, ChatBookmarkList into ChatPanel
- Add export buttons (Markdown) and bookmarks panel toggle in header
- Wire branch-on-edit: branchConversation called when editing message with subsequent replies
- Add scroll-to-message support in ChatMessageList via virtualizer.scrollToIndex
- Show GitBranch icon for branch conversations in ChatConversationList