feat(merge): diff Merge opens the 3-pane editor + conflict overview ruler

- The Merge button in the Diff window now hands a conflicting merge to the in-app
  3-pane editor (MergeModal routes 'conflict' through RequestConflictResolution,
  the same seam Approve uses) instead of dead-ending on a conflict message.
- Add a conflict overview ruler right of the Result pane: a proportional map of
  every conflict in the file, recolored by resolved state, click a tick to jump —
  so conflicts are findable in long files without scrolling.
- New MergeResolvedEdgeBrush token + conflictMap en/de key. Ui 128 + Loc 16 green.

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

@@ -28,10 +28,17 @@ 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; }
 
+    /// 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)
     {
         _worker = worker;
@@ -96,9 +103,21 @@ public sealed partial class MergeModalViewModel : ViewModelBase
                     });
                     break;
                 case "conflict":
-                    HasConflict = true;
-                    ConflictFiles = result.ConflictFiles;
-                    ErrorMessage = Loc.T("vm.merge.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");
+                    }
                     break;
                 case "blocked":
                     ErrorMessage = Loc.T("vm.merge.blocked", result.ErrorMessage ?? "");