dsh-subagent-router: DeepSeek Harness Subagent Model Routing Plugin

The `dsh-subagent-router` plugin provides DeepSeek Harness with more flexible subagent model selection. It uses the `subagent_model` tool to allow explicitly specifying the Provider, Model, and output limit at invocation time, or to apply the built-in `model:"auto"` automatic model-selection strategy. This strategy includes task tiering, health awareness, and failure escalation. The plugin also offers `subagent_models` model directory lookup and `subagent_recommend` intelligent recommendation features. It is simple to install and takes effect immediately, primarily addressing the limited flexibility of standard routing inheritance.