dsh-image-subagent: Enabling Pure Text Main Models to Accept Image Attachments

`dsh-image-subagent` is a tool in the DSH plugin ecosystem designed to address the issue of handling image attachments in pure-text main models (v0.2.0, MIT License). This plugin bypasses the image access control gate by removing the `inputModalities` declaration from the pure-text route and projects images as explicit text placeholders carrying complete `attachmentId` and object paths. Instead of directly reading images, the main model delegates to a vision-capable subagent (e.g., `subagent_observer`), which uses the `read_image` tool to access attachments and return descriptions, thereby enabling image-text interaction under pure-text routing. The plugin requires Node >=20 and `@deepseek-ai/dsh >=0.1.1-rc.2` (opti