Introducing the dsh-collapsible-models Plugin

`dsh-collapsible-models` is a DeepSeek Harness plugin designed to address the issue of excessively long model provider lists. It takes over the model selector by registering shadow entries and supports collapsing and expanding model groups. Users can interact by clicking provider names or using keyboard navigation, with accessibility and multi-language environments supported. Collapsed states are only retained for the current session. The plugin is open source and requires no build step.