feat(ui): add merge-target dropdown and merge-all controls to planning detail
- Add SubtaskDiffDto and CombinedDiffResultDto to PlanningDtos.cs
- Extend IWorkerClient with 5 planning merge methods and 5 events
- Implement methods and hub subscriptions on WorkerClient
- Add Status and WorktreeState to SubtaskRowViewModel
- Add MergeTargetBranches, SelectedMergeTarget, CanMergeAll,
MergeAllDisabledReason, MergeAllError, RecomputeCanMergeAll,
MergeAllCommand, ReviewCombinedDiffCommand (Task 14 TODO)
to DetailsIslandViewModel
- Add planning merge section to DetailsIslandView.axaml
(merge target ComboBox + buttons + error label), gated on
Task.IsPlanningParent
- Add 4 xUnit tests covering CanMergeAll logic and DTO shape
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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