dsh-usage-meter: DeepSeek Harness Model Usage Dashboard Plugin

`dsh-usage-meter` is a local usage-metrics plugin for DSH agent development. It leverages the official Cordis service to monitor sessions and provides aggregated metrics, such as token counts and cache hit rates, along with time-based trend charts on the settings page. It supports viewing by multiple dimensions, including service provider and model, and offers API integration with `cc-switch` synchronization. The plugin has no external dependencies and stores all data locally, making it suitable for developers to monitor costs, evaluate efficiency, and analyze call patterns.