The model sometimes wrote the skill's invitation sentence without calling
propose_action, so users read "click below" with no card. The plugin kind
now registers its invitation, which only an accepted call's result carries:
"Card shown below your answer. Do not restate it. End with at most one short
invitation in the user’s language to click Enable below; do not say the
plugin is already on." The skill drops its invitation sentence and says the
call's result tells how to end the answer. A test runs a real turn and checks
that the accepted result carries the invitation and a refused call does not;
the plugin Web fixture is refreshed for the new skill and result text.