Collapse TryCompleteParentAsync (planning -> Done) and
TryAdvanceImprovementParentAsync (improvement -> WaitingForReview) into one
TryAdvanceParentAsync that surfaces any WaitingForChildren parent for review
once all children are terminal. Planning parents no longer auto-complete.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>