refactor(agent-loop): simplify message machine

docs/core-data-structures/core.zh.md

@@ -422,46 +422,12 @@ type SessionEvent<T extends SessionEventType = SessionEventType> = {
 
 源码：[`packages/core/agent/src/types.ts`](../../packages/core/agent/src/types.ts)
 
-```ts type-equiv
-/**
- * Which inbox queue a {@link Agent.send} item joins:
- * - `next-turn` — the item becomes its own turn, claimed at a turn boundary.
- * - `next-step` — during prompt admission or an open turn, the item stages for
- *   the next safe step boundary; otherwise it is promoted per its `wakeup`
- *   flag.
- */
-type SendTarget = 'next-turn' | 'next-step'
-```
-
 ```ts type-equiv
 /** Resolved inbox placement reported when an accepted message is enqueued. */
 type InboxPlacement = 'queued' | 'steering'
 ```
 
-```ts type-equiv
-/**
- * Options for the unified {@link Agent.send} primitive over the
- * (`target` × `wakeup`) matrix. Named presets: {@link Agent.followup}
- * (`next-turn`/wakeup), {@link Agent.steer} (`next-step`/wakeup), and
- * {@link Agent.inject} (`next-step`/no-wakeup).
- *
- * The object is complete so routing policy is explicit.
- */
-interface SendOptions {
-  /** Queue the item joins. */
-  target: SendTarget
-  /**
-   * Whether this item makes the model run: wake a parked driver (`next-turn`)
-   * or force a continuation step (`next-step` while running). A `false`
-   * `next-turn` item queues without waking; a `false`
-   * `next-step` item attaches durable context without forcing another step
-   * (the injection preset).
-   */
-  wakeup: boolean
-}
-```
-
-固定预设的别名方法自带 `target` 与 `wakeup`；其已有标识的 `UserMessage` 会携带角色、内容与 provenance。投递方法不会返回其 `MessageId`，但该 id 在这条消息的各个 `agent/inbox/*` 事件中保持稳定。注入绕过两个 FIFO，从不出现在这些事件中。
+投递方法接收已有标识的 `UserMessage`，由它携带角色、内容与 provenance。投递方法不会返回其 `MessageId`，但该 id 在这条消息的各个 `agent/inbox/*` 事件中保持稳定。注入绕过两个 FIFO，从不出现在这些事件中。
 
 ```ts type-equiv
 /** Options for {@link Agent.cancel}. */
@@ -482,10 +448,10 @@ type AgentCancelCause =
   | { readonly kind: 'parent' }
 ```
 
-`Agent` 是覆盖公开活跃 agent 契约的接口。具体驱动器拥有 `followup`/`steer`/`inject` 别名方法，并将它们经由 `send` 的（`target` × `wakeup`）矩阵路由。
+`Agent` 是覆盖公开活跃 agent 契约的接口。具体驱动器实现 `followup`、`steer` 和 `inject`；路由策略仍为驱动器私有。
 
 ```ts type-equiv
-/** Public live-agent handle with aliases over the unified delivery primitive. */
+/** Public live-agent handle. */
 interface Agent {
   /** The single identity shared with {@link session}. */
   readonly id: SessionId
@@ -504,28 +470,6 @@ interface Agent {
   /** Agent-scoped context; its contributions are agent-local, unwind on disposal, and reject registration afterward. */
   readonly ctx: Context
 
-  /**
-   * The unified delivery primitive over the (`target` × `wakeup`) matrix.
-   * It routes the caller's typed content and source as follows:
-   *
-   * - `next-turn` queues an item that becomes the sole ordinary message of its
-   *   own FIFO-ordered turn; `wakeup:true` wakes a
-   *   parked driver, while `wakeup:false` queues without waking.
-   * - `next-step` with `wakeup:true` stages steering during prompt admission
-   *   or an open turn; outside that window it falls back to a woken
-   *   `next-turn`.
-   * - `next-step` with `wakeup:false` injects durable model-facing context
-   *   without running the model: admission or an open turn stages it for the
-   *   next safe log position, while an injection outside that window appends
-   *   immediately without opening a turn. If admission closes without a turn,
-   *   a context-only boundary appends immediately; context staged beside
-   *   steering remains pending with it.
-   * The agent publishes or queues the identified frozen message as-is.
-   * @param message - identified model-facing content and its producer provenance.
-   * @param options - target queue and wakeup decision.
-   */
-  send(message: UserMessage, options: SendOptions): void
-
   /**
    * Clear queued and steering work — unless `keepInbox` — and abort the active
    * turn. An effective call first emits `agent/cancel-requested` with the
@@ -541,16 +485,14 @@ interface Agent {
   whenIdle(): Promise<void>
 
   /**
-   * Queue an ordinary follow-up turn and wake the driver — the
-   * `next-turn`/wakeup preset of {@link send}. The item becomes the sole
-   * ordinary message of its own turn.
+   * Queue an ordinary follow-up turn and wake the driver. The item becomes the
+   * sole ordinary message of its own turn.
    * @param message - identified prompt content and its producer provenance.
    */
   followup(message: UserMessage): void
 
   /**
-   * Submit steering during prompt admission or an open turn — the
-   * `next-step`/wakeup preset of {@link send}. It stages for the next steering
+   * Submit steering during prompt admission or an open turn. It stages for the next steering
    * checkpoint before a request or stop decision. If the activity fails before
    * that boundary, the remainder stays staged without waking the agent; retry
    * or a later prompt takes it. Outside that window steering falls back to a