Function cmd_poll 

pub async fn cmd_poll(
    room_id: &str,
    token: &str,
    since: Option<String>,
    mentions_only: bool,
) -> Result<()>

One-shot poll subcommand: read messages since cursor, print as NDJSON, update cursor.

Reads the caller’s username from the session token file. When mentions_only is true, only messages that @mention the caller’s username are printed (cursor still advances past all messages).