reply holds only the clean assistant text from the most recently
completed turn. It is written atomically when Submit returns and cleared
(as a documented side-effect) when a new prompt is submitted. Stat
reports the real byte length and bumps Qid.Vers on each change, so
polling via stat size > 0 is the intended signal that a reply is ready.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>