Restore writeTranscript bool return instead of empty-string sentinel

writeTranscript returned a bool on main ("was a transcript written"). The compact-transcript change switched it to return the pointer filename (or "" when nothing was written) so the pointer could target transcript.jsonl. That pointer move was deferred — the metadata pointer is always full.jsonl now — so the string return carried no more information than the bool, just an empty-string sentinel. Restore the bool signature; the caller uses paths.TranscriptFileName directly.