Function agents_to_seed 

fn agents_to_seed(
    registered: &[String],
    is_live: impl Fn(&str) -> bool,
) -> Vec<String>

The agents to put in the subscription when it is created: every locally registered agent that a live pane still holds, lowercased and deduplicated. Registered agents used to be missed. On an isolated channel the subscriber is created lazily at muxbus.login, with an empty set, and an agent that registered earlier (while there was no subscriber to tell) joined only on its next input turn, so nothing reached it until then.