dsh-deepseek-vision: Adding Image Input Routing for DeepSeek Harness's Pure Text Model

dsh-deepseek-vision is a DSH plugin designed to enhance pure-text DeepSeek models with image input capabilities, addressing the gap where existing models cannot directly process visual content. The plugin registers an independent `DeepSeek + Vision` route, supporting multiple input sources such as chat window image pasting, screenshots, and tool-returned images. Its core mechanism employs a分流 processing approach: if the model directory declares visual support (e.g., deepseek-v4-flash-vision-exp), images are directed straight to the native visual endpoint; otherwise, a configurable VL model is called to convert the image description into text before passing it to DeepSeek for processing. The plugin offers an LRU cache-based description reuse mechanism to avoid duplicate billing, retains original images in session logs, and supports OpenAI-style gateways and