Function dw_agent_dispatch_mode 

pub fn dw_agent_dispatch_mode(
    get_env: impl Fn(&str) -> Option<String>,
) -> DwAgentDispatch

Resolve how dw.agent nodes execute. GREENTIC_AW_DISPATCH=nats routes them over the out-of-process agentic NATS path; anything else (incl. unset) keeps the in-process path — zero regression by default. Pure over get_env for testability (mirrors should_serve_agentic_inproc).