dsh-git-nexus: Single-Plugin Git and GitHub Integration for DeepSeek Harness

DeepSeek Harness (DSH) introduces a new plugin, `dsh-git-nexus`, designed to address the fragmented Git management workflow in local development. The plugin unifies SCM changes, branch operations, GitHub integration, workflow tracking, and file browsing into a single web panel. It supports core features such as viewing diffs, committing, switching branches, and pulling or pushing, and securely logs in to GitHub via OAuth Device Flow. The plugin executes commands in isolated subprocesses and restricts output to enhance security. Developers need to mount a local path for installation, making it suitable for development scenarios that require maintaining context continuity in a web interface.