Ctrl+G already opened $EDITOR with the current draft, but used
open_in_editor(validate_and_handle=False), which only loaded the saved text
back into the input area — the user still had to press Enter. The TUI's
Ctrl+G (openEditor) submits the draft on a clean exit. Since CLI submission
is driven by the custom Enter keybinding (not the buffer accept_handler),
validate_and_handle can't route through it; instead chain a done-callback on
the editor Task that calls the new _submit_editor_buffer(), which mirrors the
Enter handler's idle/queue/slash branches and drops an empty save.
