Add @ path reference to DeepSeek Harness input box with dsh-at-file

### 正式翻译（符合技术文档规范）： `dsh-at-file` is a DeepSeek Harness Web plugin maintained by omdsh-dev. It allows users to search the workspace by typing `@` in the input box and insert the path of a file or directory. Starting from version 0.3.0, it only attaches workspace references and no longer embeds the full content of files into the prompt. This article verifies the installation commands, selectors, filtering rules and precautions against the community directory page and GitHub README.