v0.28.0

Added

• Ops history: external approval reference (#854). Every mutating
  tool takes an optional approval parameter,
  {reference, approved_by, approved_at}, naming a sign-off made
  outside mureo that the change carries out, and
  mureo_state_action_log_append accepts entry.approval. It is stored
  on the action_log entry and on the decision records the change
  writes (including the decision of a call that wrote no entry, which an
  approval records even without a reason), returned
  as approval on ops history actions and decisions, and shown on the
  dashboard as "Approved by X on D (ref R)". mureo records and shows it
  and verifies nothing about it; it is never required. A malformed value
  is refused with invalid_args before the platform is called. Old
  STATE.json files read and write unchanged. See docs/ops-history.md.