feat(interactive): New session button for ad-hoc ConPTY sessions

Adds a 'New session' header button in Mission Control that opens a folder picker
and starts a task-less embedded ConPTY session in the chosen directory. ConPtyPaneViewModel
TaskId is now nullable (ad-hoc panes have no task and are never deduped) with a
CreateAdHoc factory; the view does the picking, the VM stays picker-agnostic.

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

@@ -341,6 +341,71 @@ public class MissionControlViewModelTests : IDisposable
         Assert.Empty(vm.Panes);
     }
 
+    private sealed class ThrowingAdHocLaunchSpecWorker : StubWorkerClient
+    {
+        public override Task<LaunchSpec> GetAdHocLaunchSpecAsync(string directory, CancellationToken ct = default)
+            => throw new InvalidOperationException("bad directory");
+    }
+
+    [Fact]
+    public async Task OpenAdHocConPtySessionAsync_AddsPane_TitleFromDirectoryLeaf()
+    {
+        var worker = new FakeWorker();
+        using var vm = BuildVm(worker);
+
+        await vm.OpenAdHocConPtySessionAsync(Path.Combine("C:", "Some", "Path", "MyProject"));
+
+        Assert.Single(vm.ConPtySessions);
+        Assert.Null(vm.ConPtySessions[0].TaskId);
+        Assert.Equal("MyProject", vm.ConPtySessions[0].DisplayTitle);
+        Assert.Single(vm.Panes);
+        Assert.Same(vm.ConPtySessions[0], vm.Panes[0]);
+    }
+
+    [Fact]
+    public async Task OpenAdHocConPtySessionAsync_TwoOpens_NeverDeduped_YieldsTwoPanes()
+    {
+        var worker = new FakeWorker();
+        using var vm = BuildVm(worker);
+
+        var dir = Path.Combine("C:", "Some", "Path");
+        await vm.OpenAdHocConPtySessionAsync(dir);
+        await vm.OpenAdHocConPtySessionAsync(dir);
+
+        Assert.Equal(2, vm.ConPtySessions.Count);
+        Assert.Equal(2, vm.Panes.Count);
+        Assert.All(vm.ConPtySessions, s => Assert.Null(s.TaskId));
+    }
+
+    [Fact]
+    public async Task OpenAdHocConPtySessionAsync_WorkerThrows_RaisesErrorReported_NoPaneAdded()
+    {
+        var worker = new ThrowingAdHocLaunchSpecWorker();
+        using var vm = BuildVm(worker);
+        string? error = null;
+        vm.ErrorReported += msg => error = msg;
+
+        await vm.OpenAdHocConPtySessionAsync(Path.Combine("C:", "Some", "Path"));
+
+        Assert.Empty(vm.ConPtySessions);
+        Assert.Empty(vm.Panes);
+        Assert.NotNull(error);
+    }
+
+    [Fact]
+    public async Task CloseConPtySession_RemovesAdHocPane_FromConPtySessionsAndPanes()
+    {
+        var worker = new FakeWorker();
+        using var vm = BuildVm(worker);
+        await vm.OpenAdHocConPtySessionAsync(Path.Combine("C:", "Some", "Path"));
+        var pane = vm.ConPtySessions[0];
+
+        pane.CloseCommand.Execute(null);
+
+        Assert.Empty(vm.ConPtySessions);
+        Assert.Empty(vm.Panes);
+    }
+
     [Fact]
     public void ToggleLayoutCommand_FlipsIsFocusMode()
     {