I imagine most harnesses should have a way to do this today, if they don't, get a new one. OpenCode i.e. is highly customizable, Claude and VS Code both support a ton as well including custom agents (though unclear if you can create custom top-level in claude-code)
https://opencode.ai/docs/agents/
https://code.claude.com/docs/en/sub-agents
https://code.visualstudio.com/docs/agent-customization/custo...