Download transcripts (auto-captions) for videos from a YouTube channel within a date range, as timestamped markdown linked back to the source video. Use when the user wants to grab transcripts/subtitles/captions from a channel, e.g. "download transcripts for all AI Daily Brief videos from July 2026", "get the captions from this channel for last month", "pull transcripts between two dates". Requires yt-dlp and ffmpeg installed.
