dsh-preset-switch: In-Session Agent Preset Switching for DeepSeek Harness

The DeepSeek Harness (DSH) plugin `dsh-preset-switch` enables users to dynamically switch Agent Presets within the same session without restarting it. It uses extended commands and an HTTP interface to bypass the official blank-session lock restriction. The plugin provides `/preset` menu selection, keyboard shortcuts, and toggle controls, allowing developers to flexibly test different roles and toolchain configurations. Install it using `--profile web`, and note that switching may trigger tool-call compatibility warnings.