dsh-ark-plan: Activating DeepSeek v4 Flash on the Volcano Ark Plan API

The `dsh-ark-plan` plugin for DeepSeek Harness is designed to simplify activating DeepSeek v4 Flash through the Volcano Ark Plan API without creating a custom provider. It reuses the built-in `deepseek` provider and points to the Ark Plan API by overriding `baseURL`, enabling a “budget lane” setup. After installation, users need to configure separate `DEEPSEEK_API_KEY` and `ARK_API_KEY` values and switch to the `deepseek` provider to use it. The plugin injects `reasoningEffort: max` by default and supports 1M context; users should avoid installing the outdated `ark` provider and ensure proper handling of secrets.