dsh-md-picker: DeepSeek Harness Doc-to-Markdown Plugin

`dsh-md-picker` is a DeepSeek Harness (DSH) plugin that addresses inadequate document support in the official attachment pipeline by converting documents to Markdown and staging them locally. Core features include image injection, verbatim text staging, multi-format document conversion, SHA-256 deduplication, and secure directory protection. The plugin depends on Node 20+ and supports drag-and-drop uploading, progress display, and a receipt mechanism that avoids consuming context, making it suitable for developers to process documents while preventing context overflow.