feat(ui): open merge-helper ConPTY tile from selection

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

@@ -406,6 +406,65 @@ public class MissionControlViewModelTests : IDisposable
         Assert.Empty(vm.Panes);
     }
 
+    private sealed class ThrowingMergeHelperLaunchSpecWorker : StubWorkerClient
+    {
+        public override Task<LaunchSpec> GetMergeHelperLaunchSpecAsync(IReadOnlyList<string> taskIds, string? listId, CancellationToken ct = default)
+            => throw new InvalidOperationException("spec failed");
+    }
+
+    [Fact]
+    public async Task OpenMergeHelperConPtySessionAsync_AddsAdHocPane()
+    {
+        var worker = new FakeWorker();
+        using var vm = BuildVm(worker);
+
+        await vm.OpenMergeHelperConPtySessionAsync(null, new[] { "t1", "t2" });
+
+        Assert.Single(vm.ConPtySessions);
+        Assert.Null(vm.ConPtySessions[0].TaskId);
+        Assert.Single(vm.Panes);
+        Assert.Same(vm.ConPtySessions[0], vm.Panes[0]);
+    }
+
+    [Fact]
+    public async Task OpenMergeHelperConPtySessionAsync_TwoRuns_NeverDeduped()
+    {
+        var worker = new FakeWorker();
+        using var vm = BuildVm(worker);
+
+        await vm.OpenMergeHelperConPtySessionAsync(null, new[] { "t1" });
+        await vm.OpenMergeHelperConPtySessionAsync(null, new[] { "t1" });
+
+        Assert.Equal(2, vm.ConPtySessions.Count);
+    }
+
+    [Fact]
+    public async Task OpenMergeHelperConPtySessionAsync_EmptySelection_NoPane()
+    {
+        var worker = new FakeWorker();
+        using var vm = BuildVm(worker);
+
+        await vm.OpenMergeHelperConPtySessionAsync(null, Array.Empty<string>());
+
+        Assert.Empty(vm.ConPtySessions);
+        Assert.Empty(vm.Panes);
+    }
+
+    [Fact]
+    public async Task OpenMergeHelperConPtySessionAsync_WorkerThrows_RaisesErrorReported_NoPaneAdded()
+    {
+        var worker = new ThrowingMergeHelperLaunchSpecWorker();
+        using var vm = BuildVm(worker);
+        string? error = null;
+        vm.ErrorReported += msg => error = msg;
+
+        await vm.OpenMergeHelperConPtySessionAsync(null, new[] { "t1" });
+
+        Assert.Empty(vm.ConPtySessions);
+        Assert.Empty(vm.Panes);
+        Assert.NotNull(error);
+    }
+
     [Fact]
     public void ToggleLayoutCommand_FlipsIsFocusMode()
     {