Use this to proactively reach the user when they may be away from the terminal:
a long-running task finished, a build/training run succeeded or failed, or you
have hit something that needs their decision before you can continue. Do NOT use
it for routine progress chatter or to echo an answer they are clearly watching.

Args:
    message: The notification body. Keep it one line and lead with what they'd
        act on (e.g. "train run failed: OOM at step 1200").
    title: Optional short bold title (e.g. the node or job name).
    tag: Which configured recipient group to send to (default "default").

Returns "ok" on success, or an error string describing why it failed.