refactor(merge): single IMergeCoordinator replaces the 5 conflict seams

The RequestConflictResolution Func was declared on 5 VMs and hand-threaded shell->details->merge-section->diff->merge-modal. Replaced with a DI-singleton IMergeCoordinator (MergeCoordinator holder; shell wires its Handler at composition, breaking the shell<->island cycle). Invokers (MergeModal, DetailsIsland, WorktreesOverview) depend on the interface; the two pass-through VMs (DiffModal, MergeSection) drop the seam entirely. No behavior change; conflict-seam + batch tests rewired to assert via the coordinator.

src/ClaudeDo.Ui/Views/WindowDialogService.cs

@@ -87,8 +87,6 @@ public sealed class WindowDialogService : IDialogService
                 var mergeDlg = new MergeModalView { DataContext = mergeVm };
                 await mergeDlg.ShowDialog(_owner);
             };
-            vm.RequestConflictResolution = (taskId, target) =>
-                shell.RequestConflictResolutionAsync(taskId, target);
         }
         await dlg.ShowDialog(_owner);
     }