dsh-car-loan: Car Loan Monthly Payment Calculator

`dsh-car-loan` is an open-source plugin for DeepSeek Harness (DSH) that performs precise local calculation of monthly car loan payments. It is implemented purely in Node.js, has no network dependencies, and requires the `@deepseek-ai/cordis` and `@deepseek-ai/dsh-tools` dependencies. Install it with `dsh plugin add github:uckkk/dsh-car-loan`. After installation, simply call the `car_loan` tool to perform the calculation.