fix(agent-loop): park empty turn batches, keep max-tokens sticky, and settle headless at idle

- turn boundary now returns false for an empty admitted batch (claimed
  input removed before the wake) instead of opening a turn and spending a
  model call on nothing; the step boundary already had the symmetric guard.
- a max-token step stays sticky when steering or injected work continues
  the turn: a later completed step no longer downgrades the outcome,
  matching the TurnEndReasonMap contract.
- session/queue wire schema accepts the context placement (previously the
  zod union rejected injected-context snapshots wholesale and the client
  silently dropped the whole frame); schema tests cover all placements.
- headless runs settle at whole-agent idle instead of the first turn/end,
  honoring the one-shot idle-to-idle contract.
- flush JSDoc names the real callers (checkpoint policy, goal-session,
  teardown, self-flushing consumers); apiproxy zh README loses its stale
  duplicate history section; ACP note/README record the delivered error
  rejection and turnless-cancelled behaviors.

packages/core/session/src/index.ts

@@ -928,10 +928,11 @@ export class SessionStore extends Service {
   /**
    * Dispatch the awaited `session/flush` durability checkpoint for `session`,
    * with the carrier captured at {@link enter}. THE flush entry point: the
-   * store owns the carrier, so callers (the loop's turn-end checkpoint, idle
-   * injection, teardown drains) must come through here rather than dispatch a
-   * raw `ctx.parallel('session/flush', …)` — one owner, one spelling, and the
-   * scoped-dispatch invariant can pin it.
+   * store owns the carrier, so callers (the checkpoint policy's per-request
+   * barrier, goal-session's idle checkpoint, teardown drains, and consumers
+   * that flush themselves before reading storage) must come through here
+   * rather than dispatch a raw `ctx.parallel('session/flush', …)` — one owner,
+   * one spelling, and the scoped-dispatch invariant can pin it.
    * @param session - the session whose buffered events must reach durable storage.
    * @returns whether at least one durability listener participated, after every
    *   listener has settled successfully.