Resolves a pending action by either applying or discarding it.

• action is required:
  • "apply" persists / submits the pending action.
  • "discard" rejects the pending action.
• reason is required: one short complete sentence explaining why, starting with a capital letter and ending with a period.
• extra (optional) is free-form metadata passed to the resolving tool. When the pending action is a plan-approval gate, supply extra.title (kebab/PascalCase slug for the approved plan filename). For preview-style pending actions (e.g. ast_edit), extra is unused.

Valid whenever a pending action exists — either a preview-style staging (e.g. ast_edit) or a long-lived approval gate. Call fails with an error when no pending action exists.