dsh-second-opinion: Asynchronous Second-Model Review for DeepSeek Harness

`dsh-second-opinion` is a DeepSeek Harness plugin that provides an asynchronous second-layer review mechanism. It does not block the primary model, supports concurrent and serial control, and offers both advisory and automatic reconsideration modes. Through environment variables and configuration, users can specify a dedicated review model and anchor review results to the conversation. This mechanism implements tool isolation and state persistence, effectively improving the quality of agent responses; the initial version is primarily optimized for the Web profile.