dsh-session-navigator: DeepSeek Harness Session Navigation and Thumbnail Plugin

`dsh-session-navigator` is a client-side plugin designed for the DeepSeek Harness Web UI to improve the efficiency of locating turns in long conversations. It adds a full-height navigation rail and proportional scale to the right edge of the chat stream, visualizes user turns, and provides hover previews, smooth navigation, and quick actions (such as copying and exporting). It uses a zero-intrusion design, requires no modifications to the official core package, is easy to install, and does not involve build steps, significantly reducing the operational cost of finding information across many turns.