Replaces the direct EF Status write in PlanningMergeOrchestrator with
_state.ApproveReviewAsync, enforcing the TaskStateService invariant as
sole owner of Status writes. Handles the improvement-parent path where
TaskMergeService already approved the parent's own worktree during the
drain (status == Done on entry → still success). If the parent was
concurrently cancelled, the transition guard rejects the approve,
PlanningCompleted is not broadcast, and the cancelled status is
preserved. ApproveReviewAsync now also sets FinishedAt.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
