Remove the SessionDetail.messages stripping and related cache-size
change per maintainer feedback. The session-detail optimization will
follow separately after PR #58 lands with the right architectural
pattern (lightweight snapshot + separate endpoints).
This PR now contains only:
- progressPayload helpers (buildProgressLogsTail, buildProgressAssistantOutput)
- cap applied to emitLogsProgress, updateProgress, stall warning, retry error
- throttle raised 300ms -> 1000ms
- tests for the progress payload behavior