dsh-md-overlay: Markdown Preview Panel in DSH

The DSH (DeepSeek Harness) plugin `dsh-md-overlay` addresses layout and interaction challenges in previewing Markdown documents within a session by providing a floating/dockable panel and the `md_preview` tool. It supports multi-tab management, drag-to-resize width, syntax highlighting for code blocks, and table-of-contents generation, and it only reads files within the workspace. Users can trigger previews via commands or by clicking a filename. Relative-path images and LaTeX/Mermaid syntax are currently not supported.