render_prompt prefixed each recording with a '## <recorded_at>' heading.
No system prompt consumes that timestamp, and the LLM occasionally echoed
it into the final document as a heading. Join recordings with the bare
'\n\n---\n\n' separator instead; chronology is carried by recording order.
Adds a regression test pinning that no RFC3339 timestamp or '##' reaches
the prompt.
closes#8
Brummel
4dc15c5450