dsh-vision-subagent: Enabling Vision Capabilities for DeepSeek Harness Text Agents

The `dsh-vision-subagent` plugin provides visual analysis capabilities to the main conversation by delegating image processing to independent subagents running on routes such as MiniMax or Kimi. It implements context isolation, multi-turn visual reasoning, and cost separation, while supporting image pasting on the web and high-fidelity reproduction. Installation requires route configuration. Currently, it is limited to local file reading and does not support remote URLs, aiming to address context overflow issues.