dsh-typst-preview: Implementing Real-Time Typst Preview in the DeepSeek Harness Sidebar

The `dsh-typst-preview` plugin locally launches `tinymist` to integrate real-time Typst document preview into the DeepSeek Harness sidebar. It reads only the current tab's content, with no outbound network traffic or script hooks, and supports bidirectional switching between source code and the rendered page, as well as syntax highlighting. It requires manual installation and a restart, and is compatible with DSH `>=0.1.5-rc.1`.