file-brief: DeepSeek Harness Task-Level File Structure Indexing Plugin

`file-brief` is a DeepSeek Harness plugin designed to address the inefficiency of Agents repeatedly checking file formats and structures. It scans task directories to generate a Markdown document containing file metadata, such as field names, types, and sizes, along with a SQLite index. By not storing raw data, it helps protect privacy and save storage space. The plugin supports multiple file formats, including CSV, JSON, Excel, and PDF. Users can run Python scripts to create file briefs, query information, and refresh data, enabling Agents to quickly access file structure knowledge, avoid redundant reads, and improve execution efficiency.