dsh-plugin-git: Portable Git Operations and Client Interface

`dsh-plugin-git` is the standard Git integration plugin for DeepSeek Harness (DSH), supporting both Desktop and Web environments. It provides a portable Host service and Client component, and integrates code change tracking, commit graphs, and diff viewing within a sidebar. The plugin is preinstalled in Desktop and can be installed in the Web environment via the command line. It is currently in an experimental development phase and depends on the upstream `ctx.sidebarRight` service.