Optional expressiveOptional jsonWhen true, each chunk sent on the lk.transcription datastream topic is serialized
as a JSON object with text, and start_time/end_time/confidence/
start_time_offset when the captured value is a TimedString. Each object is
suffixed with a newline so subscribers can parse the stream line-by-line.
Whether expressive markup may be present in the text reaching this sink.
Evaluated per chunk, because the session latches it on the first turn that injects the markup guide — after this sink is constructed. Defaults to "never", so a session that doesn't use expressive mode publishes its transcript untouched: the strip works off the union of every provider's tag names, and an agent that legitimately writes
<break time="1s"/>should not have it silently deleted.