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.

src/ClaudeDo.Ui/Views/MissionControl/MissionControlView.axaml.cs

@@ -2,6 +2,7 @@ using System.Linq;
 using Avalonia.Controls;
 using Avalonia.Input;
 using Avalonia.Interactivity;
+using Avalonia.Platform.Storage;
 using Avalonia.VisualTree;
 using ClaudeDo.Ui.ViewModels;
 using ClaudeDo.Ui.ViewModels.Islands;
@@ -21,6 +22,23 @@ public partial class MissionControlView : UserControl
         AddHandler(DragDrop.DropEvent, OnPaneDrop);
     }
 
+    // Ad-hoc ConPTY session: the view owns the folder picker, the VM only takes the chosen path.
+    private async void OnNewSessionClicked(object? sender, RoutedEventArgs e)
+    {
+        if (DataContext is not MissionControlViewModel vm) return;
+        var topLevel = TopLevel.GetTopLevel(this);
+        if (topLevel is null) return;
+
+        var folders = await topLevel.StorageProvider.OpenFolderPickerAsync(new FolderPickerOpenOptions
+        {
+            Title = "Choose a directory",
+            AllowMultiple = false,
+        });
+        if (folders.Count == 0) return;
+
+        await vm.OpenAdHocConPtySessionAsync(folders[0].Path.LocalPath);
+    }
+
     private void OnPaneDragOver(object? sender, DragEventArgs e)
     {
         var dt = e.DataTransfer;