dsh-side-chat: DeepSeek Harness Standalone Side Chat Plugin

`dsh-side-chat` is a plugin for DeepSeek Harness (DSH) that aims to provide multithreaded conversation capabilities via Better Sidebar. It allows users to fork independent side sessions from the main session, inheriting the context while hiding the previous history, and supports streaming interactions, tool calls, and Markdown rendering. It requires a service restart after installation. Users can perform parallel reasoning or debugging in the sidebar, and session state is automatically persisted.