Function tail 

pub async fn tail(
    url: &str,
    identity: &Identity,
    session: &str,
    channel: &str,
    since_secs: Option<u64>,
) -> Res<()>

Joins channel and streams its traffic to stdout until Ctrl-C — the minimal human “watch the agents talk” view (PRD-0011 T-004; the §19 aggregation log’s smallest sibling). App-level pings keep the session inside the server’s idle-reap window.

With since_secs, the retained backlog is replayed first (PRD-0013 T-004). A dropped link (server restart / deploy) reconnects with backoff and resumes from the watermark — status goes to stderr so stdout stays a clean message stream.

Errors

Returns an error if the first connection, authentication, or join fails, or the server rejects the stream; a transport drop after a working stream reconnects instead of erroring.