fix(worker): propagate unit-merge failures instead of reporting success

A child merge that came back blocked/verify_failed/untracked_collision during a
parent/children unit merge used to vanish: DrainAsync only logged it server-side,
PlanningMergeAborted carried no reason, and ApproveReview/review_task always
reported StatusMerged for a task with children regardless of the real outcome,
so a failed unit merge left the parent stuck with no visible error.

- PlanningMergeOrchestrator.StartAsync/ContinueAsync/DrainAsync now return a
  PlanningMergeResult(Status, Reason) instead of void, and PlanningMergeAborted
  carries that reason to the UI.
- WorkerHub.ApproveReview and ExternalMcpService.ReviewTask's approve branch
  propagate the real status/reason for a parent with children instead of
  hardcoding "merged" (or masking a non-conflict failure as "conflict").
- StartAsync now requires the parent to already be WaitingForReview for
  improvement parents too, not just planning ones, so a stale caller can no
  longer trigger a partial child merge.
- HasActiveMerge now also covers the window between the last child merging and
  FinalizeParentDoneAsync completing, closing a gap where a concurrent Cancel
  could race the parent's own approve-to-Done transition.
- IslandsShellViewModel.OnPlanningMergeAborted flashes the reason via
  FlashFooterError instead of only clearing the external-merge banner.

src/ClaudeDo.Worker/CLAUDE.md

@@ -174,7 +174,9 @@ launch specs · worktrees · agents/settings/lists · reports/notes/prep · diag
 - `PlanningMergeStarted`
 - `PlanningSubtaskMerged`
 - `PlanningMergeConflict`
-- `PlanningMergeAborted`
+- `PlanningMergeAborted` (`planningTaskId, reason` — every call site passes a human-readable
+  reason now, from a real merge failure's `ErrorMessage` to a plain "Merge aborted."; the UI
+  flashes it via `FlashFooterError`)
 - `PlanningCompleted`
 - `RefineStarted`
 - `RefineFinished`