dsh-deepseek-vision-router: Add an image preprocessing router to DeepSeek Harness

`dsh-deepseek-vision-router` is a plugin for DSH (DeepSeek Harness) designed to address the limitation where text-only DeepSeek main agents cannot directly process images. It implements image preprocessing via a dedicated `deepseek-vision` route: first sending the image to a configured vision model to generate a textual description, and then submitting the description to DeepSeek for processing. During this process, the official DeepSeek adapter continues to handle the main session, reasoning, streaming, and tool calls, while the plugin only stores visual provider and model settings without touching credentials or copying endpoints. The plugin supports configuring vision models in DSH settings and selecting the "DeepSeek + Vision" mode in the model menu to enable this configuration.