Add Configuration Snapshot and One-Click Rollback to DeepSeek Harness via dsh-undo-plugin

The `dsh-undo-plugin` is a DSH community plugin maintained by lire1131, with the package name `dsh-undo-savepoint` after installation. It automatically and manually takes snapshots for configurations and user plugin codes, supports undo, redo, and rolling back to any version, and provides a PowerShell off-box tool and safe mode when Harness fails to start. This article verifies the installation command, snapshot scope and precautions based on the directory page and GitHub.