dsh-git-update-notifier: Update Management in Local Source Format

`dsh-git-update-notifier` is an update-detection plugin for DeepSeek Harness (DSH), specifically designed for source-code checkout development. It detects updates by directly comparing Git commits, avoids breaking the pnpm workspace structure, and supports resumable updates, multi-stage validation, and rollback on failure. The plugin automatically checks at midnight daily, performs catch-up checks if it is not running, and provides Web GUI interaction, retry mechanisms, and self-update capabilities. It is compatible with npx, npm, and proxy-based environments.