dsh-tax-refund: Tax Refund Estimation

`dsh-tax-refund` is a DeepSeek Harness (DSH) plugin maintained by uckkk, providing local tax estimation using pure Node.js. The plugin can calculate tax refunds without making network requests, making it suitable for scenarios that require local tax estimates. Users can install it with the `dsh plugin add` command and directly invoke the `tax_refund` tool within a DSH session to obtain detailed results.