Delete (row context menu + detail header) now gates on HasInteractiveSession
the same way SendToQueue already does, so an open ConPTY session's claude
process can't be orphaned by deleting its task out from under it.
Row removal (in-UI delete and the delta-refresh path for externally deleted
tasks) now clears SelectedTask via SelectFrom(null, "row-removed") when the
removed row was selected, so the detail pane no longer keeps showing a
deleted task with live buttons.
