Add no-progress turn detector that inspects the most recent assistant
message: if finish is 'unknown', all token counts are zero, and there
is no meaningful content beyond step-start/step-finish markers, the
turn is classified as no-progress.
Integrate the check at all three idle/completion/error continuation
points in the event handler so the loop stops cleanly with a warning
toast instead of injecting another internal prompt.
YeonGyu-Kim