Preserve Claude SDK child environment

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

@@ -8,7 +8,7 @@
 
 ## 受管环境命名空间与捕获的输出
 
-`DSH_*` 变量是归 Harness 所有的子进程事实；实现会在合并调用方显式 `env` 之前丢弃环境中已有的 `DSH_*` 名称，因此当前事实只会以有意提供的条目形式到达，每条被收集的流都通过 `CollectedOutput` 报告自身的截断与 spill 恢复状态。
+`DSH_*` 变量是归 Harness 所有的子进程事实；实现会在合并调用方显式 `env` 之前丢弃环境中已有的 `DSH_*` 名称，因此当前事实只会以有意提供的字符串条目形式到达，而显式的 `undefined` tombstone 会删除普通环境中已有的值。每条被收集的流都通过 `CollectedOutput` 报告自身的截断与 spill 恢复状态。
 
 ```ts type-equiv
 /** One environment key inside the managed {@link DSH_ENV_PREFIX} namespace. */
@@ -115,13 +115,12 @@ interface SubprocessSpawnSpec {
   signal?: AbortSignal | undefined
   /**
    * Explicit environment entries merged onto the implementation's scrubbed
-   * parent base (see `scrubbedParentEnv`), with no namespace validation:
-   * every entry is a deliberate caller opt-in, so a forwarded
-   * credential-shaped entry or a current `DSH_*` fact survives precisely
-   * because this layer merges after the scrub that drops its ambient
-   * namesake.
+   * parent base (see `scrubbedParentEnv`), with no namespace validation. A
+   * string is a deliberate caller opt-in, so a forwarded credential-shaped
+   * entry or current `DSH_*` fact survives the scrub; `undefined` is a
+   * tombstone that removes an ordinary ambient entry from the child.
    */
-  env?: Record<string, string> | undefined
+  env?: NodeJS.ProcessEnv | undefined
 }
 ```