The `plan` skill now expresses a plan's open questions, propositions and
clarifications as an ask-form field spec in a fenced json block under
`## Questions`. The backend parses it (reusing forms.valid_fields), the plan
page renders it with the same controls the conversation forms use, answers are
kept in localStorage per plan, and Submit & work drops the implement-this-plan
prompt into the composer with the decisions appended.