dsh-conversation-branches: Enable Independent Q&A Branches in the Main Conversation

The `dsh-conversation-branches` plugin is maintained by Caramel-Geass to address the need for in-depth analysis of specific segments within DeepSeek Harness. Users can select text in the main conversation to open an independent IM-style streaming Q&A branch in the right-side details pane, with support for multi-turn follow-up questions, collapsing and deletion, and data persistence. The plugin requires no additional configuration and enables non-intrusive, selection-based analysis, enhancing the flexibility of the session.