round 1: implement bracket-first manual compaction

packages/core/agent/src/types.ts

@@ -178,6 +178,20 @@ export interface Agent {
    */
   send(message: UserMessage, options: SendOptions): void
 
+  /**
+   * Reserve admission of the next ordinary turn while this agent is idle, so an
+   * operation can mutate durable history before any queued prompt derives a
+   * request from it. Already-accepted waking work has right of way, including a
+   * send whose wake is still a pending microtask. Later sends keep their
+   * ordinary placement, FIFO order, and `wakeup` facts, and
+   * {@link acceptsNextStep} stays `false`, so a waking `next-step` send becomes
+   * a queued follow-up rather than steering; cancellation and disposal may
+   * still discard them. {@link inject} is not withheld. {@link whenIdle} treats
+   * a live reservation as activity, while lifecycle teardown does not await it.
+   * @returns the idempotent release, or `undefined` when the agent is running, already reserved, or already committed to waking work.
+   */
+  reserveTurnAdmission(): (() => void) | undefined
+
   /**
    * Mutate one still-pending queued occurrence synchronously. Editing preserves
    * the message identity and queue position; removal publishes its terminal