dsh-date-calc: Date Calculation Plugin

`dsh-date-calc` is a pure Node.js date arithmetic plugin for DeepSeek Harness (DSH), maintained by uckkk. It provides local, encapsulated support for date interval calculations, date addition/subtraction, and weekday queries, reducing dependence on external services. Users can install it with the `dsh plugin add` command and then invoke the relevant tools during a session. The plugin runs with the current process's permissions, so users should review the source code to ensure security before use.