fix(ui): surface precondition hints for silently-disabled review buttons (UX-Audit #2)

Cancel now shows an amber hint + tooltip while a merge drain is in
progress (ShowMergeDrainHint mirrors IsMergeDraining). Continue,
Reset & Retry, Open Diff, Worktree, and Review Combined Diff gain
ToolTip.Tip + ToolTip.ShowOnDisabled explaining why they're locked.
No CanExecute or behavior changed, only added communication.

tests/ClaudeDo.Ui.Tests/ViewModels/DetailsIslandReviewActionsTests.cs

@@ -138,6 +138,24 @@ public class DetailsIslandReviewActionsTests : IDisposable
         Assert.False(vm.ApproveReviewCommand.CanExecute(null));
     }
 
+    [Fact]
+    public void ShowMergeDrainHint_IsTrueExactlyWhenMergeIsDraining()
+    {
+        var vm = BuildVm(new RecordingWorkerClient());
+        vm.Bind(new TaskRowViewModel { Id = "task-drain-1", Status = TaskStatus.WaitingForReview });
+
+        Assert.False(vm.ShowMergeDrainHint);
+        Assert.True(vm.CancelReviewCommand.CanExecute(null));
+
+        vm.IsMergeDraining = true;
+        Assert.True(vm.ShowMergeDrainHint);
+        Assert.False(vm.CancelReviewCommand.CanExecute(null));
+
+        vm.IsMergeDraining = false;
+        Assert.False(vm.ShowMergeDrainHint);
+        Assert.True(vm.CancelReviewCommand.CanExecute(null));
+    }
+
     private sealed class ThrowingWorkerClient : StubWorkerClient
     {
         public override bool IsConnected => true;