dsh-rollback: Conversation and File Rollback Plugin for DeepSeek Harness

`dsh-rollback` is a conversational and file-state synchronized rollback plugin for DeepSeek Harness (DSH), designed to address the issues of inefficient manual rollback and state inconsistency in agent development. The plugin supports creating checkpoints before each turn and managing historical states using a 10-turn sliding window, enabling restoration of file contents, deletion of files, and handling of newly created files. It provides both Web-based visual buttons and command-line triggering, supports change previewing and in-place truncation. The plugin requires DSH version 0.1.1-rc.2 or higher and includes a comprehensive error-handling mechanism.