dsh-sketch-reference: DeepSeek Harness Native Collaborative Visualization Plugin

The `dsh-sketch-reference` plugin integrates Excalidraw into the DeepSeek Harness workspace for visual collaboration. Users can create sketches and add annotations; the agent can read them on demand, identify relevant areas, and propose changes. Revisions are applied only after user confirmation. Installation requires DSH Web version 0.2.1-alpha and adding the plugin via the plugin manager. Data is stored locally, and automatic two-way binding is not supported.