dsh-data-mode: DeepSeek Harness Data Mode Plugin

The `dsh-data-mode` plugin for DeepSeek Harness introduces a "Data Mode" session type to DSH, focusing on read-only data analysis. It allows users to connect to databases or upload files and generate read-only SQL queries using natural language. It also provides table previews, a business knowledge base (with BM25 retrieval), and environment isolation, ensuring that queries are secure and do not affect other sessions' presets. Installation requires a Node.js >= 22.19.0 environment, and once enabled, users can interact with data by specifying the appropriate preset.