dsh-git-branch: Display the current workspace git branch in DSH Web UI

`dsh-git-branch` is a DSH plugin maintained by `hanyueqiang` and licensed under the MIT License, designed to address the unintuitive display of workspace Git branches in the DSH Web UI. The plugin displays the current workspace's Git branch information next to the Composer mode controls, supporting both normal branches (e.g., `⎇ main`) and detached HEAD states; it automatically hides if the workspace is not a Git repository. The plugin features a smart refresh mechanism that updates immediately upon plugin mounting, session/workspace switching, window focus, page visibility changes, or user clicks, and includes a 60-second polling interval as a fallback (which can be disabled via configuration). Architecturally, the Host side registers a `/git-branch` command.