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/Views/MissionControl/ConPtyPaneView.axaml

@@ -39,9 +39,15 @@
               Background="{DynamicResource ErrorTintBrush}"
               BorderBrush="{DynamicResource BloodBrush}"
               BorderThickness="0,0,0,1" Padding="12,6">
-        <TextBlock Classes="meta" Text="{Binding Terminal.StartError}"
-                   Foreground="{DynamicResource BloodBrush}"
-                   TextWrapping="Wrap" />
+        <Grid ColumnDefinitions="*,Auto">
+          <TextBlock Grid.Column="0" Classes="meta" Text="{Binding Terminal.StartError}"
+                     Foreground="{DynamicResource BloodBrush}"
+                     TextWrapping="Wrap" VerticalAlignment="Center" />
+          <Button Grid.Column="1" Classes="btn" Margin="12,0,0,0"
+                  Content="{loc:Tr missionControl.retry}"
+                  ToolTip.Tip="{loc:Tr missionControl.retryTip}"
+                  Command="{Binding RetryCommand}" />
+        </Grid>
       </Border>
 
       <!-- Embedded ConPTY terminal, with a starting overlay until the session is spawned -->