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.Localization/locales/en.json

@@ -263,6 +263,7 @@
     "detach": "Detach",
     "redock": "Re-dock",
     "windowTitle": "Mission Control",
+    "newSession": "New session",
     "clearFinished": "Clear finished",
     "empty": "No running tasks",
     "settings": "Settings",