dsh-rembg: Local Image Background Removal Plugin

`dsh-rembg` is a local image background removal plugin for DeepSeek Harness (DSH), enabling agents to process images without cloud APIs. Installation requires DSH version 0.1.7-rc.1 or later; after installing it from the command line, initial configuration must be completed in the Web GUI, including selecting the runtime mode (CPU or GPU) and installing the model. The plugin provides the `rembg` tool function, which supports removing backgrounds by specifying a model and an image path, making it suitable for local agent development scenarios without public API access.