DeepSeek Harness Session File Upload Plugin

The DeepSeek Harness (DSH) plugin provides arbitrary file upload and delivery capabilities for the web interface using a plugin-based architecture. Its core features include writing files to session directories, model awareness, and generating download cards via the `share_session_file` tool, while also supporting new UI download entry points and SSH remote workspaces. The plugin implements security mechanisms such as a sessionId whitelist and filename sanitization, with a default per-file limit of 512 MiB. Installation requires restarting DSH and is suitable for agent scenarios that need to persist or distribute files within sessions.