dsh-artifact-index: The Missing Backend for the DeepSeek Harness Sidebar

`dsh-artifact-index` is a backend plugin for the `dsh-artifacts` sidebar. It provides the artifact index JSON and file byte streams. The plugin runs on the DSH web server and serves content via GET requests, supporting configuration of the root directory, maximum file size, and security policies. It is dependency-free and secure, allowing only loopback access and not returning the `mine` field. After installation, DSH Desktop must be restarted, and functionality can be verified through logs or scripts. It resolves the issue where the sidebar cannot read the index due to the lack of a backend.