Function plan 

pub fn plan(
    env: &Environment,
    home: &Path,
    rc_already_wired: bool,
    config: Option<&ConfigPlan>,
) -> Vec<Step>

Build the ordered plan for this environment. rc_already_wired is whether the rc file already carries the marker line (checked by the caller, injected here to stay pure). config is the routing file to generate, if any — it is written before the proxy starts so the child can be handed it, since a config the proxy never loads is worse than no config at all.