dsh-file-preview: File Preview Plugin for DeepSeek Harness Workspaces

`dsh-file-preview` is a file browsing plugin for DeepSeek Harness (DSH), designed to integrate file operation capabilities. It provides features such as a file browser in the left sidebar, a list of files generated in a session, drag-and-drop path insertion, and preview support for multiple formats (Markdown, PDF, images, CSV, and code). It currently supports local source installation only and is enabled by configuring `package.json`. The plugin supports workspace isolation and text truncation (with a 5MiB limit), primarily helping developers efficiently view and analyze files within the DSH interface.