docs(subagent): qualify ACP cleanup failure

packages/subagent/subagent-acp/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/subagent/subagent-acp/README.md
-README.md: 00f084c8252c001d98d04c8ccc1dff5f14976683
-README.zh.md: 3b820805d23b39757376edc14a3d75860f2e8eae
+README.md: 93a1578ac060a0493f52da5f85372039333eb5dc
+README.zh.md: fd7da81f7202406099b815ff3122454637aa87fb

packages/subagent/subagent-acp/README.md

@@ -6,7 +6,7 @@ The ACP provider runs each subagent in a fresh subprocess and drives it as an Ag
 
 ## Start and ownership
 
-`start(request)` resolves the child's working directory, then performs `spawn` → ACP `initialize` → `newSession` before it fulfills. Fulfillment therefore means a remote session is ready and ownership has transferred to the caller. A spawn, initialization, new-session, or pre-publication cancellation failure rejects only after the subprocess has been reaped; a working-directory resolution failure rejects before anything is spawned. Non-cancellation rejections expose only fixed provider, stage, and category facts in their Error message; the original failure remains on the internal cause chain and in Host diagnostics.
+`start(request)` resolves the child's working directory, then performs `spawn` → ACP `initialize` → `newSession` before it fulfills. Fulfillment therefore means a remote session is ready and ownership has transferred to the caller. A spawn, initialization, new-session, or pre-publication cancellation failure ordinarily rejects after the subprocess is reaped; when cleanup itself rejects, ordered safe startup/teardown facts preserve both failures without claiming process exit. A working-directory resolution failure rejects before anything is spawned. Non-cancellation rejections expose only fixed provider, stage, and category facts in their Error message; the original failure remains on the internal cause chain and in Host diagnostics.
 
 The working directory is the configured `cwd` override when set, else the delegating parent session's cwd — never the server process's own cwd, because one server process serves sessions from many workspaces. The parent-derived value must be an absolute path naming a directory the harness can enter (search permission — what a subprocess cwd needs), and the same resolved path becomes both the subprocess cwd and the ACP `session/new` workspace.
 

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

@@ -6,7 +6,7 @@ ACP（Agent Client Protocol）提供方会在全新的子进程中运行每个 s
 
 ## 启动与所有权
 
-`start(request)` 先解析子 agent 的工作目录，再依次执行 `spawn` → ACP `initialize` → `newSession`，然后才兑现。因此，兑现表示远程会话已就绪，所有权也已转移给调用方。spawn 失败、初始化失败、新建会话失败或因发布前取消而失败时，只有在子进程已回收后才会拒绝；工作目录解析失败则会在尚未 spawn 任何进程时拒绝。非取消拒绝的 Error 消息只公开固定的 provider、stage 与 category 事实；原始失败仍保留在内部 cause 链和 Host 诊断中。
+`start(request)` 先解析子 agent 的工作目录，再依次执行 `spawn` → ACP `initialize` → `newSession`，然后才兑现。因此，兑现表示远程会话已就绪，所有权也已转移给调用方。spawn 失败、初始化失败、新建会话失败或因发布前取消而失败时，通常会在子进程已回收后拒绝；若清理自身也拒绝，有序的安全 startup/teardown 事实会保留两项失败，但不会宣称进程已经退出。工作目录解析失败则会在尚未 spawn 任何进程时拒绝。非取消拒绝的 Error 消息只公开固定的 provider、stage 与 category 事实；原始失败仍保留在内部 cause 链和 Host 诊断中。
 
 工作目录优先使用已配置的 `cwd` 覆盖值，否则使用执行委派的父会话 cwd，绝不使用服务器进程自身的 cwd，因为同一个服务器进程会服务来自多个工作区的会话。从父级取得的值必须是绝对路径，指向 harness 可以进入的目录（具备搜索权限，这是子进程 cwd 的要求）；解析后的同一路径同时作为子进程 cwd 和 ACP `session/new` 工作区。
 

packages/subagent/subagent-acp/src/run.ts

@@ -322,9 +322,10 @@ function terminalFailure(
 
 /**
  * Start and publish one ACP child after initialization and session creation.
- * Child failures resolve through the run result; startup and teardown failures
- * reject with fixed safe facts after process reap, retaining original causes
- * for Host observation. Disposal cancels, kills, and reaps the child.
+ * Child failures resolve through the run result. Startup rejects with fixed
+ * safe facts after provider-owned cleanup; successful cleanup proves process
+ * reap, while cleanup failure preserves both causes without claiming
+ * quiescence. Disposal cancels, kills, and reaps the child.
  * @param request - the start request; its signal is the cancellation channel.
  * @param spec - the resolved spawn spec: command/args/cwd, env, permission
  * policy, dispose graces, and the optional error sink.