Add Git-style milestone navigation bar to DeepSeek Harness sessions with dsh-milestone

`dsh-milestone` is a DeepSeek Harness Web plugin maintained by SnowCrescenter-tech. It draws a Git-style dot timeline on the right side of the conversation: each dot represents one query, and you can hover to view details such as the round, time consumption, TTFT, model and tokens, and click to jump. It supports search, bookmarks, keyboard navigation and `#msg=` deep links. The directory installation command is `dsh plugin add github:SnowCrescenter-tech/dsh-milestone`. The current version is 0.6.0, under MIT license. The plugin runs with the permissions of the current dsh process, and the search scope is limited to the loaded message windows.