/**
 * lessons-digest-layer migration (lessons-digest/AC-19..AC-22).
 *
 * Projects that ran retrospectives before the digest existed have a `lessons` layer
 * registered as always-read, and it grows by design — up to three lessons per instance.
 * In this repo it reached 90,285 bytes, 85% of everything a session read before touching
 * task work. This migration flips that registration to `on_demand: true` and registers
 * the bundled `lessons-digest` layer in its place as the always-read distillation.
 *
 * `confirm` tier, not `safe`: it changes what every future session reads. "Sessions
 * silently stopped reading the lessons file" is not a change to make on someone's behalf,
 * which is the whole distinction the tier exists to draw.
 *
 * It deliberately writes no layer *bodies*. `install-guidance` (safe tier, same doctor
 * run) copies `lessons-digest.md` in, because that is the code path that knows how to
 * detect a locally-modified layer and leave it alone. This one only ever edits
 * `index.yaml`, and never touches `lessons.md`.
 */
import type { Migration } from '../types.js';
export declare const lessonsDigestLayer: Migration;
/**
 * Bytes of a project's `lessons.md`, or null when absent.
 *
 * Exported for the migration's own test, which asserts the file is byte-identical
 * across an apply: the migration's promise is that it edits the index and nothing else.
 *
 * @param projectRoot - Absolute project root.
 */
export declare function lessonsBody(projectRoot: string): string | null;
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