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/ViewModels/Modals/DiffModalViewModel.cs

@@ -72,7 +72,6 @@ public sealed partial class DiffModalViewModel : ViewModelBase
     public string TaskTitle { get; init; } = "";
     public Func<MergeModalViewModel, Task>? ShowMergeModal { get; set; }
     public Func<MergeModalViewModel>? ResolveMergeVm { get; set; }
-    public Func<string, string, Task>? RequestConflictResolution { get; set; }
 
     public ObservableCollection<DiffFileViewModel> Files { get; } = new();
 
@@ -100,7 +99,6 @@ public sealed partial class DiffModalViewModel : ViewModelBase
     {
         if (TaskId is null || ShowMergeModal is null || ResolveMergeVm is null) return;
         var vm = ResolveMergeVm();
-        vm.RequestConflictResolution = RequestConflictResolution;
         await vm.InitializeAsync(TaskId, TaskTitle);
         await ShowMergeModal(vm);
         // The diff is stale once the worktree merged away or a conflict opened the editor.

src/ClaudeDo.Ui/ViewModels/Modals/MergeModalViewModel.cs

@@ -9,6 +9,7 @@ namespace ClaudeDo.Ui.ViewModels.Modals;
 public sealed partial class MergeModalViewModel : ViewModelBase
 {
     private readonly IWorkerClient _worker;
+    private readonly IMergeCoordinator _merge;
 
     public string TaskId { get; set; } = "";
     public string TaskTitle { get; set; } = "";
@@ -28,10 +29,6 @@ public sealed partial class MergeModalViewModel : ViewModelBase
 
     public Action? CloseAction { get; set; }
 
-    /// Set by the caller to hand a conflicting merge off to the in-app 3-pane editor
-    /// instead of dead-ending on the conflict message.
-    public Func<string, string, Task>? RequestConflictResolution { get; set; }
-
     /// True once a merge has succeeded — lets the caller (e.g. the diff window)
     /// close itself after this modal closes.
     public bool Merged { get; private set; }
@@ -39,9 +36,10 @@ public sealed partial class MergeModalViewModel : ViewModelBase
     /// True once a conflict has been handed off to the resolver — also a cue to close the diff window.
     public bool RoutedToResolver { get; private set; }
 
-    public MergeModalViewModel(IWorkerClient worker)
+    public MergeModalViewModel(IWorkerClient worker, IMergeCoordinator merge)
     {
         _worker = worker;
+        _merge = merge;
     }
 
     public async Task InitializeAsync(string taskId, string taskTitle)
@@ -103,21 +101,11 @@ public sealed partial class MergeModalViewModel : ViewModelBase
                     });
                     break;
                 case "conflict":
-                    // Hand off to the in-app 3-pane merge editor when wired (MergeTask aborted
-                    // cleanly, so the resolver re-starts the merge leaving conflicts in the tree).
-                    if (RequestConflictResolution is not null)
-                    {
-                        var branch = SelectedBranch!;
-                        RoutedToResolver = true;
-                        CloseAction?.Invoke();
-                        await RequestConflictResolution(TaskId, branch);
-                    }
-                    else
-                    {
-                        HasConflict = true;
-                        ConflictFiles = result.ConflictFiles;
-                        ErrorMessage = Loc.T("vm.merge.conflict");
-                    }
+                    // MergeTask aborted cleanly; hand the conflict to the in-app 3-pane editor,
+                    // which re-starts the merge leaving conflicts in the tree.
+                    RoutedToResolver = true;
+                    CloseAction?.Invoke();
+                    await _merge.ResolveConflictAsync(TaskId, SelectedBranch!);
                     break;
                 case "blocked":
                     ErrorMessage = Loc.T("vm.merge.blocked", result.ErrorMessage ?? "");

src/ClaudeDo.Ui/ViewModels/Modals/WorktreesOverviewModalViewModel.cs

@@ -62,6 +62,7 @@ public sealed partial class WorktreesOverviewModalViewModel : ViewModelBase
 {
     private readonly IWorkerClient _worker;
     private readonly Func<WorktreeModalViewModel> _diffVmFactory;
+    private readonly IMergeCoordinator _merge;
 
     [ObservableProperty] private string? _listIdFilter;
     [ObservableProperty] private string _title = "Worktrees";
@@ -79,9 +80,6 @@ public sealed partial class WorktreesOverviewModalViewModel : ViewModelBase
     public ObservableCollection<string> MergeTargets { get; } = new();
     public ObservableCollection<WorktreeOverviewRowViewModel> ConflictRows { get; } = new();
 
-    /// Inert seam wired by the integrator to Layer C's resolver at merge time. (taskId, targetBranch)
-    public Func<string, string, Task>? RequestConflictResolution { get; set; }
-
     public Action? CloseAction { get; set; }
     public Action<WorktreeModalViewModel>? ShowDiffAction { get; set; }
     public Action<string, string>? JumpToTaskAction { get; set; }
@@ -89,10 +87,11 @@ public sealed partial class WorktreesOverviewModalViewModel : ViewModelBase
     public Func<MergeModalViewModel>? ResolveMergeVm { get; set; }
     public Func<MergeModalViewModel, Task>? ShowMergeAction { get; set; }
 
-    public WorktreesOverviewModalViewModel(IWorkerClient worker, Func<WorktreeModalViewModel> diffVmFactory)
+    public WorktreesOverviewModalViewModel(IWorkerClient worker, Func<WorktreeModalViewModel> diffVmFactory, IMergeCoordinator merge)
     {
         _worker = worker;
         _diffVmFactory = diffVmFactory;
+        _merge = merge;
     }
 
     public void SelectRow(WorktreeOverviewRowViewModel row)
@@ -328,7 +327,7 @@ public sealed partial class WorktreesOverviewModalViewModel : ViewModelBase
     private void ResolveConflict(WorktreeOverviewRowViewModel? row)
     {
         if (row is null) return;
-        RequestConflictResolution?.Invoke(row.TaskId, SelectedTarget ?? "");
+        _ = _merge.ResolveConflictAsync(row.TaskId, SelectedTarget ?? "");
     }
 
     [RelayCommand]