dsh-undo: Persistent Undo/Redo per User Turn for DeepSeek Harness

dsh-undo is a preview undo/redo plugin for DeepSeek Harness (dsh), designed to address session rollback issues in multi-step tool tasks. It achieves precise undo by rolling back the visible interface according to real user message boundaries and restoring workspace files modified by tools during that turn. Core features include the local command `/undo` (rolling back the latest or a specified turn number), `/redo` (restoring in LIFO order), and rollback operations for message bubbles in the WebUI. The plugin records snapshots in an isolated Git directory to restore files, supporting tracked files and untracked files up to 2MiB, operating only within the Git workspace root, without interfering with the original Git state. Additionally, it terminates top-level background tasks initiated during rolled-back turns. Currently, this plugin is at version 0.3.0-preview.1, depending on