/**
 * The plan-negotiation state machine. A plan moves through:
 *
 *   no-plan → draft (via propose) → frozen (via approve)
 *
 * Revise keeps draft as draft. Reopen moves frozen back to draft. The state
 * can only reach frozen via explicit approve from draft — never inferred.
 *
 * Pure (no I/O) so it is unit-tested directly.
 */
export type PlanState = "no-plan" | "draft" | "frozen";
export type PlanAction = "propose" | "revise" | "approve" | "reopen";
export declare function nextPlanState(current: PlanState, action: PlanAction): PlanState;