dsh-plugin-image-tools: Three tools to add image capabilities to DSH Web GUI

# Introduction When using the Web GUI of DeepSeek Harness (DSH) to create agents, there are three image-related scenarios that have always been difficult to handle: 1. The model wants you to select an image from a set of candidates (e.g., choosing a book cover), but images cannot be placed in the tabs; the model can only print the file path for you to open manually. 2. The model generates an image and wants to mix it with text in the response body, but there is no channel for this either. 3. You send an image into the chat, and a text-only model adapter directly reports `UNSUPPORTED_CONTENT`, interrupting the turn. dsh-plugin-image-tools is a DSH plugin designed for these three scenarios: image selection card, inline images in the response, and blind model receiving images. Three tools, each handling a specific task, with zero configuration.