dsh-sampling-sliders: Add a real-time adjustable sampling parameters panel for DSH

In agent development, this situation often arises: the default temperature or maxTokens of the model is unsuitable, and one wishes to temporarily adjust them to compare results. However, this usually requires modifying the configuration file and restarting the service every time. If you only want to adjust the temperature from its default value to 0.2 for a single conversation, current solutions either require modifying global settings that affect all sessions or lack an interface to make changes. The philosophy behind DeepSeek Harness (DSH) is "everything is a plugin," making this kind of requirement a perfect fit for a plugin. The dsh-sampling-sliders plugin introduced below addresses this: it adds a "Sampling" button to the input bar. Upon clicking, sliders appear to adjust the temperature and maxTokens for each individual session.