Whether an ASSISTANT row holds client-side output, not agent output.
Set for both ! and !! shell output, which render through
AssistantMessage without invoking the agent. Lets callers ask whether the
agent produced anything in a thread rather than trusting the row type —
see _store_has_server_output and the /copy scan in app.py.
Never set on a restored transcript: shell output reaches thread state as a
<user_shell_command> human message, not as an assistant turn.
