dsh-sla-calc: SLA Availability Calculation

The DeepSeek Harness (DSH) plugin `dsh-sla-calc` is designed for local Service Level Agreement (SLA) availability calculation. Implemented purely in Node.js with no network dependencies, it is suitable for offline environments. Users can install it via the provided command and use the `sla_calc` tool to obtain detailed availability calculation results. The plugin is licensed under the MIT License, addressing users' need for precise, localized operational metrics.