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/Services/IMergeCoordinator.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Threading.Tasks;
+
+namespace ClaudeDo.Ui.Services;
+
+/// <summary>
+/// Single entry point for handing a conflicting merge to the in-app 3-pane resolver.
+/// Replaces the per-VM <c>RequestConflictResolution</c> Func seams that used to be
+/// hand-threaded shell → details → merge-section → diff → merge-modal. The shell wires
+/// <see cref="MergeCoordinator.Handler"/> once at composition; invokers depend only on
+/// this interface (injected via DI).
+/// </summary>
+public interface IMergeCoordinator
+{
+    Task ResolveConflictAsync(string taskId, string targetBranch);
+}
+
+/// <summary>
+/// DI singleton holding the resolver entry. The holder breaks the shell↔island construction
+/// cycle: islands depend on the interface, the shell sets <see cref="Handler"/> after it is built.
+/// </summary>
+public sealed class MergeCoordinator : IMergeCoordinator
+{
+    /// Set once at composition to the shell's resolver entry. Null (headless/tests) ⇒ no-op.
+    public Func<string, string, Task>? Handler { get; set; }
+
+    public Task ResolveConflictAsync(string taskId, string targetBranch) =>
+        Handler?.Invoke(taskId, targetBranch) ?? Task.CompletedTask;
+}