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.
Task rows now drive a hand-built pointer-capture drag instead of
DragDrop.DoDragDropAsync: armed on press, begins past a 4px threshold so a
plain click still selects. The ghost follows the screen cursor across windows;
on release the action is decided by what is under the cursor -- over the
Mission Control window queues the task (geometric DragHitTest, no OLE drop),
over another row in the same user list reorders, anywhere else cancels and
restores the row. Drag starts from any list kind (drag-to-queue everywhere)
but reorder-on-drop stays gated on CanReorder. Removes the obsolete OLE
TaskRowFormat path from both the source and MissionControlView (pane
PaneFormat reorder is untouched).
mika kuns