dsh-review-checkout: DeepSeek Harness Plugin for Conversation File Modification Review

The `dsh-review-checkout` plugin for DeepSeek Harness (DSH) aims to address the lack of visualization and rollback support for Agent file operations. Using a local snapshot mechanism, it generates cards resembling the official "Edited" cards for each `write`/`edit` operation and provides a review view with syntax highlighting, actual line numbers, and restore actions. The plugin supports undo, opening files at a specific line number, session statistics, and custom themes, filling a gap in official DSH regarding file history tracking and rollback.