ruaibeite/dsh-undo

Turn-level file undo for tool writes and edits: each file is snapshotted before the write lands, and the model (an undo tool), the human (a /undo command) or a button in the assistant message action row can roll back every file a turn changed. Snapshots are written in-process rather than by shelling out, so it needs no extra sandbox permission in sessions confined to workspace-write.