dsh-repurchase: DeepSeek Harness Repurchase Rate Calculation Plugin

The `dsh-repurchase` plugin, released by DeepSeek Harness (DSH) and maintained by uckkk, is open-sourced under the MIT license. It is designed to calculate repurchase rates in offline environments without network access, performing precise calculations based on specified parameters and returning the results. Implemented in pure Node.js with no network dependencies, it requires `@deepseek-ai/cordis ^4.0.1` and `@deepseek-ai/dsh-tools >=0.1.0-rc.6`. Users can install it via a command and invoke the registered tool during a session.