Function send_message 

pub async fn send_message(
    url: &str,
    identity: &Identity,
    session: &str,
    channel: &str,
    text: &str,
) -> Res<()>

Joins channel and posts one message as the authenticated session, awaiting the server’s ack (sends are server-confirmed) — the CLI-as-a-human-client post (PRD-0011 T-004).

Errors

Returns an error if the connection, authentication, join, or send is rejected or times out.