dsh-engine-switch: Route DSH Sessions to Pluggable Engines Based on Presets

`dsh-engine-switch` is a workflow plugin for DeepSeek Harness (DSH), designed to address the need for different AI engines (such as Claude Code) across different sessions within the same client. It does not provide concrete engine implementations; instead, it uses a proxy pattern to dynamically switch the engine driving top-level sessions based on presets, falling back to DeepSeek by default. Its core features include selecting an engine by preset, supporting instant switching at runtime, automatically loading presets, and automatically restoring the engine when a session is resumed. It decouples engine selection from session logic, while subagents always use the DeepSeek loop. After installation, it must be enabled and registered in the configuration, and it supports specifying engines via preset files or configuration mappings, requiring Node.js ^22.19.0 or higher.