DSH Message Navigator: DeepSeek Harness Message Navigation Plugin

The `dsh-message-navigator` plugin provides a ChatGPT-like message navigation experience for DeepSeek Harness (DSH). It generates message index markers to the right of the left sidebar, supporting click-to-navigate, keyboard controls, and screen reader accessibility. It also offers hover previews, displaying user messages (multi-line bold) and assistant responses (gray text), with support for previews of attachments such as images and files. The plugin automatically loads historical messages to complete the index, adapts to light and dark themes as well as narrow screens, and does not replace the existing conversation view. The installation command is `dsh plugin --profile web add github:yx-yinhe/dsh-message-navigator#v1.0.2`.