redo 2

DSH undo/rollback system: snapshot config files on change, undo/redo the last action from the WebUI or by chat, and roll back broken plugin trees without reinstalling. Works even when DSH fails to boot (offline CLI + GUI).

User-side undo/redo for DeepSeek Harness (dsh): the /undo command rolls the transcript back to before the last message and /redo restores it.