dsh-plugin-undo: DeepSeek Harness In-Place Session Undo Plugin

`dsh-plugin-undo` is a DeepSeek Harness client plugin for in-place conversation undo in the current session. It modifies the session surface to remove trailing turns from the model history and collapse them into a header, without forking the session or modifying project files. Users can enter `/undo` or `/undo n` while the session is idle to undo the most recent turn or multiple turns. The plugin is suitable for quickly correcting the direction of a conversation within a single session flow, but only supports undoing consecutive trailing turns.