dsh-git-chain: Git Commit Chain Graph and Branch-Switching Plugin for DSH

`dsh-git-chain` is a client plugin for DeepSeek Harness (DSH) designed to enhance the Git history viewing experience through a visual swimlane diagram. The plugin renders repository history as SVG and supports First-parent mainline views, range filtering, and instant search. It also provides commit details, diff viewing, and safe branch switching, automatically checking risks such as conflicts before switching. The plugin combines frontend rendering with backend Git CLI execution, making it suitable for code review and debugging in DSH environments. It requires specific environment prerequisites and must be installed for use.