dsh-reference-checker: Add reference authenticity check to DeepSeek Harness

dsh-reference-checker is an MIT-licensed plugin for DeepSeek Harness (DSH) designed to simplify reference validation during academic paper writing. Through the `reference_checker` model tool, it supports reading files (such as PDF, BibTeX, and TeX) or pasting text to automatically and concurrently query three major public data sources: Crossref, OpenAlex, and arXiv. It can determine the status of each reference (e.g., "found" or "partial") and generate various normalized citations in styles including APA, IEEE, and GB/T 7714 based on the input style. The plugin defaults to checking the first 50 references (with a limit of 200), requires Node.js >= 22, and has no native dependencies.