dsh-whois-plugin: Adding an RDAP Domain Query Tool to DeepSeek Harness

`dsh-whois-plugin` is a plugin for DeepSeek Harness (DSH) designed to add a `whois` tool to models for querying domain registration information. It uses the modern RDAP protocol rather than parsing traditional WHOIS text, requires no external binaries or complex dependencies, supports authoritative queries, and caches results for 24 hours. Once installed, a model can directly call the `whois` function to retrieve registrar, status, and entity information. The plugin only requires outbound HTTPS connections and provides structured data for all country-code top-level domains (ccTLDs) except those that do not have RDAP entries.