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.App/Program.cs

@@ -116,6 +116,10 @@ sealed class Program
             return new UpdateCheckService(releases, version);
         });
 
+        // Conflict-merge coordinator: single seam the shell wires to its resolver entry.
+        sc.AddSingleton<MergeCoordinator>();
+        sc.AddSingleton<IMergeCoordinator>(sp => sp.GetRequiredService<MergeCoordinator>());
+
         // ViewModels
         sc.AddTransient<WorktreeModalViewModel>();
         sc.AddTransient<Func<WorktreeModalViewModel>>(sp => () => sp.GetRequiredService<WorktreeModalViewModel>());
@@ -152,12 +156,14 @@ sealed class Program
                 sp.GetRequiredService<IDbContextFactory<ClaudeDoDbContext>>(),
                 sp.GetRequiredService<IWorkerClient>(),
                 sp,
-                sp.GetRequiredService<INotesApi>()));
+                sp.GetRequiredService<INotesApi>(),
+                sp.GetRequiredService<IMergeCoordinator>()));
         sc.AddSingleton<IslandsShellViewModel>(sp =>
         {
             var shell = ActivatorUtilities.CreateInstance<IslandsShellViewModel>(sp);
             shell.ConflictResolverFactory =
                 sp.GetRequiredService<Func<string, ClaudeDo.Ui.ViewModels.Conflicts.ConflictResolverViewModel>>();
+            sp.GetRequiredService<MergeCoordinator>().Handler = shell.RequestConflictResolutionAsync;
             return shell;
         });