dsh-local-file-upload: Uploading Local Files in Web Composer

`dsh-local-file-upload` is a client plugin for DeepSeek Harness designed to address the issue of uploading local files to the Session workspace in Web Composer scenarios. The plugin supports uploading multiple files via a toolbar button or paste (10 MiB per file), automatically saves uploaded files to the `.dsh-uploads/` directory, and generates reference markers. It enforces security restrictions by allowing only local transfer and rejecting symbolic links. Uploaded files must be cleaned up manually, and it is recommended to add them to `.gitignore`.