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>

src/ClaudeDo.Ui/Services/PlanningDtos.cs

@@ -16,3 +16,19 @@ public sealed record PlanningSessionResumeInfo(
     string WorkingDir,
     string ClaudeSessionId,
     string McpConfigPath);
+
+public sealed record SubtaskDiffDto(
+    string SubtaskId,
+    string Title,
+    string BranchName,
+    string BaseCommit,
+    string HeadCommit,
+    string? DiffStat,
+    string UnifiedDiff);
+
+public sealed record CombinedDiffResultDto(
+    bool Success,
+    string? IntegrationBranch,
+    string? UnifiedDiff,
+    string? FirstConflictSubtaskId,
+    IReadOnlyList<string>? ConflictedFiles);