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

@@ -32,6 +32,12 @@
                    LetterSpacing="1.4" VerticalAlignment="Center" />
         <StackPanel Grid.Column="1" Orientation="Horizontal" Spacing="8"
                     VerticalAlignment="Center">
+          <Button Classes="icon-btn"
+                  Click="OnNewSessionClicked"
+                  ToolTip.Tip="{loc:Tr missionControl.newSession}">
+            <PathIcon Data="{StaticResource Icon.Plus}" Width="15" Height="15"
+                      Foreground="{DynamicResource TextMuteBrush}"/>
+          </Button>
           <Button Classes="icon-btn"
                   Command="{Binding ToggleLayoutCommand}"
                   ToolTip.Tip="{Binding LayoutToggleLabel}">