dsh-reasoning: A Codex-Style Reasoning Command for DeepSeek Harness

The `dsh-reasoning` plugin provides keyboard-driven switching of reasoning effort levels for DeepSeek Harness Web. Users can quickly select modes such as Off/Low/High/Max via the `/reasoning` command or keyboard shortcuts, without using a mouse. The plugin is client-side only and does not rely on persistent configuration, ensuring session isolation and operational safety while improving interaction efficiency for keyboard users.