Drop scratch-only compatibility paths and legacy test scaffolding now that
SlateDB-backed state is authoritative. This removes scratch file fallbacks,
updates docs and UI labels, and moves tests onto durable store-backed helpers.
Complete the remaining event coverage from the events-as-source-of-truth plan.
Add response and failure-signature snapshots to stage.completed,
enrich retro.started and retro.completed with prompt/response data,
and remove the stale script field from stage.started.
Also update the internal event and run-directory docs so they match
current event payloads and derivation rules.
