subagent: seed inherited policy events at creation

The parent implementation introduced sandboxMode and approvalPolicy as generic SessionHeader fields, then propagated those fields through both persistence backends, session-query indexes, collision checks, policy-specific seed-boundary folds, catalogs, and a broad test matrix. That storage plane is unnecessary: Session already accepts a validated constructor seed, and persistence captures that seed when the session is announced before committing its first batch.

Capture each parent override synchronously at delegation, append source-tagged sandbox/mode and approval/policy records after the optional fork prefix, and create the child with that combined seed. Keeping header.seedLength at the original fork-prefix length preserves lineage while ordinary last-event-wins folds make the inherited records outrank stale parent history and remain subordinate to later child switches. Unswitched parents still stamp nothing, so children continue to follow deployment defaults.

Remove the generic header fields and every persistence/query/schema branch built around them. Collapse the inheritance suite from ten leaking scenarios to four owned-context cases covering real filesystem confinement, stale fork precedence, delegation-time capture, and the no-override path. The assembled headless snapshot now asserts the persisted inheritance event directly.

This keeps the security behavior while restoring policy ownership to the existing event log and deleting the speculative durability machinery that the original tests did not exercise.

packages/subagent/subagent-inprocess/README.zh.md

@@ -18,8 +18,6 @@
 
 子 agent 会获得父 agent 的工作目录／会话谱系；除非 `request.agentOptions` 覆盖，否则还会继承父 agent 的提供方、模型和输出 token 上限。它获得全新的扁平注册作用域：父级所有权不会导入父 agent 的工具限制，也不会建立权限子集。
 
-子 agent 还会继承父 agent 的会话策略覆盖项。驱动器在自己的第一个 await 之前同步捕获 `ctx.sandboxPolicy.overrideOf(parent.session)` 与 `ctx.approval.overrideOf(parent.session)`——委派时刻即快照点，因此与异步的子 agent 创建过程赛跑的父 agent 切换属于父 agent 的未来——并把捕获值作为创建元数据带入子 agent 不可变的 `SessionHeader`（`sandboxMode`/`approvalPolicy`），从会话存在的那一刻起就具备持久性：任何监听器顺序都不可能饿死该基线，任何崩溃窗口也不可能丢失它，包括空闲时的 SessionStart 式注入在任何提示词轮次开启之前就持久化一个完整轮次的情况。两个服务均以可选方式消费：未挂载它们的组合照旧进行无策略委派。只复制覆盖链，因此未切换过的父 agent 不写入任何基线，子 agent 继续跟随实时部署默认值；`overrideOf` 只折叠初始内容边界之后的事件，因此 fork 初始内容携带的陈旧切换已被基线所涵盖，而子 agent 自己之后的切换仍优先于基线。嵌套按构造即可组合：每次捕获解析的都是发起委派的会话自身的覆盖链（参见[设计原理](../../../.agents/notes/implemented/feature/2026-07-25-subagent-policy-inheritance.md)）。
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 ## 取消与所有权
 
 必需的请求信号同时覆盖启动阶段和实时运行。发布前，`AgentCreationTransaction` 会观察该信号、回滚并拒绝。工厂返回前会移除仅用于创建阶段的监听器；驱动器随即再次检查信号，然后安装最小化的实时运行监听器，从而消除交接竞态。发布后，中止会取消子 agent。