dsh-plugin-modality-fallback: Modality Fallback Routing for Agent Sessions

`dsh-plugin-modality-fallback` is a plugin for DeepSeek Harness (DSH) that addresses the issue of DSH forcibly switching the entire session model when the session model does not support image input. By detecting missing modality support before a request, it allows a single request to be routed to a fallback model with the required capability while keeping the session's original model configuration unchanged. It runs as a Cordis plugin without modifying core code, and currently supports fallback routing for the `image` modality only.