DSH In-Convo Mode Change

The `dsh-in-convo-mode-change` plugin transforms the preset label in the DeepSeek Harness conversation header into an interactive switcher. It allows users to change presets (such as Standard, Code, etc.) directly during an ongoing conversation, with changes taking effect starting from the next turn. Installation requires running the specified command and restarting the server. The plugin extends the client slot mechanism to provide runtime protection during conversation, preventing mid-tool-call changes to the parent scope chain.