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/ViewModels/MissionControlViewModel.cs

@@ -1,5 +1,6 @@
 using System.Collections.ObjectModel;
 using System.Collections.Specialized;
+using System.IO;
 using System.Linq;
 using CommunityToolkit.Mvvm.ComponentModel;
 using CommunityToolkit.Mvvm.Input;
@@ -265,6 +266,30 @@ public sealed partial class MissionControlViewModel : ViewModelBase, IDisposable
         }
     }
 
+    // Ad-hoc (task-less) ConPTY session in a user-chosen directory. Never deduped — every call
+    // opens a fresh pane, unlike the task-based OpenConPtySessionAsync above.
+    public async System.Threading.Tasks.Task OpenAdHocConPtySessionAsync(string directory)
+    {
+        if (string.IsNullOrEmpty(directory)) return;
+
+        var title = Path.GetFileName(directory.TrimEnd('\\', '/'));
+        if (string.IsNullOrEmpty(title)) title = directory;
+
+        try
+        {
+            var spec = await _worker.GetAdHocLaunchSpecAsync(directory);
+            var descriptor = new TerminalLaunchDescriptor(spec.Cwd, spec.Exe, spec.Args, spec.Env);
+            var pane = ConPtyPaneViewModel.CreateAdHoc(title, descriptor);
+            pane.ErrorReported += OnConPtyPaneError;
+            pane.CloseRequested += CloseConPtySession;
+            ConPtySessions.Add(pane);
+        }
+        catch (Exception ex)
+        {
+            ErrorReported?.Invoke(Loc.T("missionControl.conptyLaunchFailed", ex.Message));
+        }
+    }
+
     private void OnConPtyPaneError(string message) => ErrorReported?.Invoke(message);
 
     private void CloseConPtySession(ConPtyPaneViewModel pane)