subagent: carry inherited policy overrides in the child session header

Review fix (ds-review-bot critical #2 on #623): the first-turn event stamp
had a durability hole no turn anchoring can close — an idle SessionStart-
style injection persists a complete one-shot turn before any prompt turn
opens, so a crash in that window left a resumable-looking child with no
inherited policy, falling back to a possibly wider deployment default.

The captured overrides now ride the child's creation meta into its
immutable SessionHeader (sandboxMode/approvalPolicy, neutral strings at the
session boundary — the delegationDepth precedent), durable from the moment
the session exists: no listener ordering can starve the baseline and no
crash window can lose it. overrideOf(session) on both policy services
resolves fold(events past header.seedLength) ?? header baseline, validating
against the closed vocabulary on read; stampOverride and the prompt-submit
listener machinery are deleted. The header field rides both persistence
backends (JSONL header line; SQLite sessions columns, SCHEMA_VERSION 11 —
pre-release, no migration). pty-local reads through overrideOf so PTY
spawns see the baseline too.

Red-first: header-durability-before-any-turn test (the injection crash
window shape), baseline/seed-boundary/closed-vocabulary contract tests in
both service suites; the real-wall suite (race, veto, fork stale-seed,
grandchild) re-anchored on header assertions and green. The Agent Note's
Alternatives now records the superseded event-stamping iteration with the
review evidence; bilingual docs updated.

packages/sandbox/sandbox-policy/README.zh.md

@@ -19,7 +19,7 @@
 - `ctx.sandboxPolicy.defaultMode`／`ctx.sandboxPolicy.workspaceRoot`：`resolve()` 使用的部署默认值与回退根。
 - `effectiveSandboxMode(events)`：会话 `sandbox/mode` 事件的纯 fold（最后一次切换胜出，没有则为 `undefined`），在 `resolve()` 内使用。
 - `setSandboxMode(session, mode)`：逐会话覆盖的唯一写入路径：恰好追加一条 `sandbox/mode` 事件。切换本身就是事件；不会在带外修改模式。
-- `ctx.sandboxPolicy.overrideOf(session)`／`ctx.sandboxPolicy.stampOverride(child, mode)`：委派继承的两半：仅折叠本身（绝不包含部署默认值），以及通过 `setSandboxMode` 写入捕获的覆盖项，子 agent 已折叠出该值时跳过。进程内 subagent 驱动器在委派时捕获，并在子 agent 的第一个轮次内盖章，使发起委派的父级收紧后的模式约束其子 agent（参见[设计原理](../../../.agents/notes/implemented/feature/2026-07-25-subagent-policy-inheritance.md)）。
+- `ctx.sandboxPolicy.overrideOf(session)`：会话的覆盖链，绝不包含部署默认值：先折叠会话自己的切换（`SessionHeader.seedLength` 之后的事件），否则取会话头中继承的 `sandboxMode` 委派基线；读取时按封闭词汇校验（遇到词汇之外的值即抛出异常——这是一条持久边界）。进程内 subagent 驱动器在委派时捕获该值，并写入每个子 agent 创建时的会话头，使发起委派的父级收紧后的模式约束其子 agent，且不存在任何第一轮次的时序窗口（参见[设计原理](../../../.agents/notes/implemented/feature/2026-07-25-subagent-policy-inheritance.md)）。
 - `SANDBOX_MODES`：所有模式，用于选项展示与运行时验证。
 
 可选的 `./invariant` 配套组件会拒绝伪造的持久 `sandbox/mode` 事件，只要其值不在该封闭词汇中；Session 与其配套组件拥有周围的存储与轮次封闭规则。