dsh-submodel-change: Subagent Model Selector

The DeepSeek Harness plugin `dsh-submodel-change` lets users manually choose a model when the AI invokes subagents, addressing the need to distinguish between reasoning and summarization models in complex tasks. The plugin has zero dependencies and works only for `provider: spawn`. It supports GUI popup selection, reusing the previous selection, and nested inheritance. Installation requires restarting Desktop and supports automatic scripts or bundle methods. Configuration options allow customizing the reuse strategy and adding models. By using official extension points to intercept requests and inject model routing into the root session popup, the plugin enables fine-grained control over subtask execution under multi-model strategies.