dsh-frame-rate: Frame Rate Calculation Tool

Developers using DeepSeek Harness (DSH) can use the `dsh-frame-rate` plugin to address the need for precise frame rates in media tasks. Maintained by istone, it is a lightweight, pure Node implementation with no network dependencies. By registering the `frame_rate` tool, it supports local parameter calculations, avoiding the network overhead of relying on external APIs or errors from manual calculations. Developers only need to run `dsh plugin add` to install it and can invoke it directly in a session.