dsh-delete-message: DeepSeek Harness Message-Level Deletion Plugin

DeepSeek Harness (DSH) provides the `dsh-delete-message` plugin, designed to address context pollution in agent development by supporting message-level content deletion. Covering all message types, the plugin removes messages from context via the host contract and hides them in the frontend, supporting fine-grained deletion at the “step-level” and “entire-window” levels, as well as a bilingual (Chinese/English) and native user experience. Installation requires a full process restart and relies on host coupling, primarily helping developers clean up context quickly and reduce debugging costs.