dsh-ad-roi: Ad ROI Calculation Plugin

The `dsh-ad-roi` plugin for DeepSeek Harness (DSH) adopts a plugin-based architecture to provide convenient ROI calculation capabilities for ad delivery scenarios. Maintained by `uckkk`, the plugin supports precise calculation through parameters and returns detailed results. As a pure Node.js-based online tool, it has no network dependencies, runs with the permissions of the current process, and currently provides only core ROI functionality. Developers can install it via command and invoke it directly in a session, making it suitable for scenarios that require rapid assessment of ad effectiveness.