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/host/apiproxy/README.i18n.yaml

@@ -3,4 +3,4 @@
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/host/apiproxy/README.md
 README.md: 0b8761f57556d0076f45e2d5ab735acd092cb253
-README.zh.md: 094ce99900eeb788536cd98b7fc464cbc57fd0b9
+README.zh.md: 983bad4e3cc3d8c2ed8664d1088b2c7a2f700ef4

packages/host/apiproxy/README.zh.md

@@ -12,8 +12,6 @@
 
 首个回答认领待处理请求之前，系统会对照该请求校验问题响应。多选题的回答项可以同时携带 `selected` 中的请求选项标签与非空 `custom` 文本；单选题的回答项必须二选一。标签重复、标签未知、id 不匹配、批次不完整以及自定义文本为空都会以 `bad-response` 拒绝。
 
-`session.history` 会读取已附加 Session 的内存状态，或通过持久化检查冷日志，而不会恢复或发布 agent（智能体），然后按追加来源的消息边界分页。`maxMessages` 统计以追加方式进入 surface 的 `user/message`、`assistant/message` 和 `steering/message` 事件，因此仅供模型使用的替换副本不占用配额。每一页仍是一段连续的原始事件区间，从而让压缩（compaction）的仅日志溯源信息与引用它的替换留在同一页。
-
 `session.history` 按追加来源的消息边界分页：`maxMessages` 统计以追加方式进入 surface 的 `user/message` 和 `assistant/message` 事件，因此仅供模型使用的替换副本不占用配额。每一页仍是一段连续的原始事件区间，从而让压缩（compaction）的仅日志溯源信息与引用它的替换留在同一页。
 
 `session.history` 的尾页（不带 `beforeSeq`）额外携带一个可选的 `projections` 块——`ctx.sessionProjections`（`@deepseek-ai/dsh-session-projection`）上每个已注册单元的水位线快照，`asOfSeq` = 这些值共同反映到的最后一个事件 seq（空日志为 `-1`）。网关还订阅注册表的变更流，为每个状态发生变化的单元铸造一个 `session/projection` mux 帧（`{sessionId, key, value, seq}`——实时推送状态，绝不入日志；客户端按 seq 高者胜维护一个按会话的通用值仓）。载体不持有任何领域知识（每个值在注册表内部已过其单元自己的 schema；协议 schema 对 `values`/`value` 保持宽松）；loadOlder 页永不携带该块，未装注册表的组合则两个面都不提供。

packages/host/apiproxy/src/api/events.schema.ts

@@ -54,7 +54,7 @@ export const muxFrameSchema = z.discriminatedUnion('type', [
     sessionId: sessionIdSchema,
     items: z.array(z.object({
       id: messageIdSchema,
-      placement: z.union([z.literal('queued'), z.literal('steering')]),
+      placement: z.union([z.literal('queued'), z.literal('steering'), z.literal('context')]),
       message: messageSchema,
     })),
   }),

packages/host/apiproxy/tests/rpc-schemas.spec.ts

@@ -455,6 +455,17 @@ describe('events frame schemas', () => {
     }
   })
 
+  it('accepts every queue placement and rejects unknown placements', () => {
+    const item = (placement: string) => ({ type: 'session/queue', sessionId: 's', items: [{
+      id: 'm', placement,
+      message: { id: 'm', role: 'user', content: [], source: { kind: 'user' } },
+    }] })
+    for (const placement of ['queued', 'steering', 'context']) {
+      expect(() => muxFrameSchema.parse(item(placement))).not.toThrow()
+    }
+    expect(() => muxFrameSchema.parse(item('bogus'))).toThrow()
+  })
+
   it('rejects a queue snapshot with malformed items', () => {
     expect(() => muxFrameSchema.parse({ type: 'session/queue', sessionId: 's', items: 'x' })).toThrow()
     expect(() => muxFrameSchema.parse({ type: 'session/queue', sessionId: 's', items: [{ id: '', role: 'user', content: [], source: { kind: 'user' } }] })).toThrow()