fix(ui): keep ConPTY sessions alive across Mission Control layout rebuilds

TerminalView kills its child process on any logical-tree detach unless
BeginReparent() suppressed it, and RebuildOverviewGrid() detaches every
existing pane whenever one is added - so opening a second interactive
session killed the first. Focus-mode tab switches had the same defect.

PtyTerminalSession now puts the control in permanent reparent mode after
launch (teardown stays explicit via ConPtyPaneViewModel.Dispose -> Kill),
and the new ConPtyPaneHost reparents each pane's single long-lived view
across grid rebuilds and tab switches instead of letting the DataTemplate
instantiate a dead replacement.

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

@@ -1,11 +1,13 @@
 using System;
 using System.Collections.Specialized;
 using System.ComponentModel;
+using System.Linq;
 using Avalonia.Controls;
 using Avalonia.Interactivity;
 using Avalonia.Layout;
 using Avalonia.Media;
 using Avalonia.Platform.Storage;
+using Avalonia.VisualTree;
 using ClaudeDo.Ui.ViewModels;
 
 namespace ClaudeDo.Ui.Views.MissionControl;
@@ -73,6 +75,18 @@ public partial class MissionControlView : UserControl
             RebuildOverviewGrid();
         else if (e.PropertyName == nameof(MissionControlViewModel.HasQueued))
             UpdateQueueColumn();
+        else if (e.PropertyName == nameof(MissionControlViewModel.IsFocusMode))
+            // The now-visible layout's hosts must re-steal their pane views from the hidden
+            // one (see ConPtyPaneHost) — posted so it runs after the visibility change and
+            // the TabControl's content presenter have gone through a layout pass.
+            Avalonia.Threading.Dispatcher.UIThread.Post(ReclaimVisiblePaneHosts,
+                Avalonia.Threading.DispatcherPriority.Loaded);
+    }
+
+    private void ReclaimVisiblePaneHosts()
+    {
+        foreach (var host in this.GetVisualDescendants().OfType<ConPtyPaneHost>())
+            host.Reclaim();
     }
 
     // The queue column is Pixel-widthed (so the splitter can drag it) — when nothing is