dsh-upload-plugin: DeepSeek Harness File and Image Attachment Plugin

The DeepSeek Harness plugin `dsh-upload-plugin` addresses the lack of attachment functionality in the native editor. It allows users to upload images (as native drafts) and files (via context injection) directly through toolbar buttons or commands, without manually pasting paths. Additionally, the plugin supports browsing upload history, configuring the model's Vision capability, and setting the reasoning level. Installation requires running the command under the `web` profile, making it suitable for scenarios where frequent interaction with documents or images is needed during conversations.