dsh-cosplay: Add a global role switch to DSH Agent

dsh-cosplay is an MIT-licensed plugin designed specifically for DSH (DeepSeek Harness), aiming to enable global switch-based roleplay for Agents while preserving core capabilities such as writing code and researching information. The plugin controls the role layer via a global switch; when enabled, the Agent converses in the tone and behavior of the selected character, while disabling it immediately restores the default personality. Its character cards are compatible with SillyTavern v2 specifications, support standard JSON import and export, and include a built-in default character named "Blue Big Fat Fish." Users can use natural language to instruct the Agent to automatically search for settings and generate character cards. The plugin offers two thinking styles: neutral and role-playing. The role-playing style is only applicable to DeepSeek v4 and may impact task capabilities. Installation requires ensuring that pnpm is in the PATH.