fix(subagent): inherit parent policy overrides in continuable children

A continuable background child (the default backgroundMode for both
delegation tools) never received the parent session's explicit
sandbox/approval overrides: materialization applied only child
composition, so a danger-full-access parent produced workspace-write
children whose every out-of-workspace operation raised an approval
prompt.

Move the one-shot driver's capture/append pair into the shared
child-agent module (captureDelegatedPolicyOverrides /
appendDelegatedPolicyOverrides) and call it from both paths:
startContinuable captures before its first await, only fresh
materialization appends the source-tagged events (after any fork seed),
and a cold resume replays the persisted delegation events instead of
re-capturing the parent.

Adds the continuable inheritance unit suite, the ACP snapshot scenario
subagent-continuable-inheritance (fails without the fix), the
continuable policy-inheritance Agent Note, and the seam-level README
contract, with bilingual counterparts.

Fixes #1692

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

@@ -20,7 +20,7 @@
 
 该结果边界成立，是因为提供方拥有从发布到完全停稳的隔离子 agent 生命周期。在该生命周期内提交的 steering（中途引导）属于子运行；提供方不会声称输出只归初始 follow-up 所有。
 
-当组合中挂载了可选的沙箱策略或审批服务时，驱动器会在创建子 agent 前对父级的显式会话覆盖项获取快照，并在未发布的设置阶段追加一条带来源标记的事件，使其位于所有 fork 历史之后、会话发布之前。它绝不复制部署默认值或一次性授权；子 agent 后续的切换仍然优先。参见[策略继承决策](../../../.agents/notes/implemented/feature/2026-07-25-subagent-policy-inheritance.md)。
+驱动器通过共享的子 agent 辅助函数应用该 seam 的[委派策略继承](../subagent/README.md#delegated-policy-inheritance)：它会在创建子 agent 前捕获父级的显式沙箱／审批覆盖项，并在未发布的设置阶段追加带来源标记的事件，使其位于所有 fork 历史之后、会话发布之前。参见[策略继承决策](../../../.agents/notes/implemented/feature/2026-07-25-subagent-policy-inheritance.md)。
 
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