fix(ui): gate Mission Control submit-for-review, add retry, fix event leak

Submit for Review is now disabled while a ConPTY pane is starting, has
failed to launch, or has already exited, and MissionControlViewModel
guards against a rapid double-click racing two SubmitTaskForReviewAsync
calls. A failed launch no longer permanently occupies its TaskId dedupe
slot -- a Retry button re-fetches the launch spec and restarts the pane
in place. CloseConPtySession/Dispose now also unsubscribe
SubmitForReviewRequested, matching the other pane event handlers. Also
fixes the pre-existing nullable-dereference warning in
IslandsShellViewModel.SyncInteractiveSessionChips.

src/ClaudeDo.Ui/ViewModels/IslandsShellViewModel.cs

@@ -473,7 +473,7 @@ public sealed partial class IslandsShellViewModel : ViewModelBase, IDisposable
 
     private void SyncInteractiveSessionChips()
     {
-        if (MissionControl is null) return;
+        if (MissionControl is null || Tasks is null) return;
         Tasks.SyncInteractiveSessions(
             MissionControl.ConPtySessions
                 .Select(s => s.TaskId)