AdwAlertDialog with folder picker (Gtk.FileDialog, defaults to the
active workspace folder or home), editable name prefilled from the
folder, and a first-command row offering Shell plus every agent
whose binary is on PATH (binaries from the adapter registry) plus a
custom command field. Enter confirms, Esc cancels, Create is
suggested. Confirming runs workspace.create + pane.spawn like
'signaltty new' and focuses the new pane. Shared by Ctrl+Shift+N,
the sidebar + and the empty state.