Merge origin/master: port the SDK stack onto the subprocess seam

Master's #660 replaced dsh-subagent-subprocess with the dsh-subprocess
capability seam (ctx.subprocess + scrubbedParentEnv, tree-scoped teardown)
and moved subagent-acp onto it. Convergence for this branch's packages:

- The shared out-of-process provider vocabulary this branch had grown in
  the deleted library (NO_START_CAPABILITIES, assertPositiveFinite, cwd
  resolution, settleRunResult, subprocessRunHandle) moves into the subagent
  seam package as out-of-process.ts — it enforces subagent-seam contracts,
  not process mechanics, and both out-of-process backends now import it
  from there (subagent-acp keeps master's shape otherwise).
- subagent-sdk spawns THROUGH the SDK client (the subprocess README's
  documented exception for SDK-managed transports) and now applies the
  seam's scrubbedParentEnv() + explicit-env merge in place of the deleted
  buildChildEnv.
- sdk-client inlines the EOF→SIGTERM→SIGKILL ladder as private helpers (it
  runs outside any harness context, so it cannot ride ctx.subprocess).
- The child harness fixture gains the now-required dsh-subprocess-local
  entry for bash-local; the fixture cordis.yml keeps exercising the
  shipped provider default.

packages/subagent/README.zh.md

@@ -10,11 +10,10 @@ subagent seam 允许 agent（智能体）把工作委派给子 agent。与 [bash
 | `subagent-inprocess/` | 共享进程内运行驱动器（不提供提供方；每次运行使用一个清理 effect） | 无 |
 | `subagent-spawn/` | 进程内后端：全新的子 agent | （注册到 `ctx.subagents`） |
 | `subagent-fork/` | 进程内后端：以父 agent 已完成轮次的前缀作为初始内容的子 agent | （注册到 `ctx.subagents`） |
-| `subagent-subprocess/` | 共享进程外机制：环境变量清理、dispose（资源释放）阶梯、cwd 解析、隔离配置目录（纯库；不注册任何内容） | 无 |
 | `subagent-acp/` | 进程外后端：在派生子进程中运行并通过 ACP（Agent Client Protocol）驱动的子 agent | （注册到 `ctx.subagents`） |
 | `subagent-sdk/` | 进程外后端：在派生子进程中运行的子 harness 运行时，经 TypeScript SDK 客户端走 stdio JSON-RPC 驱动 | （注册到 `ctx.subagents`） |
 | `tool-subagent/` | 面向模型的 `subagent` 委派工具，基于 `ctx.subagents` | （注册到 `ctx.tools`） |
 
-接口位于 `subagent/subagent/`。进程内 `subagent-spawn` / `subagent-fork` 后端共享 `subagent-inprocess` 驱动器（一个自身不提供提供方的库：两者都依赖它，彼此不依赖），进程外 `subagent-acp` / `subagent-sdk` 后端则构建于 `subagent-subprocess` 库之上（凭据环境变量清理、dispose 阶梯、子进程 cwd 解析、隔离配置目录）。测试只用包内 fixture（测试前置数据）替换子 agent 边界。
+接口位于 `subagent/subagent/`。进程内 `subagent-spawn` / `subagent-fork` 后端共享 `subagent-inprocess` 驱动器（一个自身不提供提供方的库：两者都依赖它，彼此不依赖），进程外 `subagent-acp` / `subagent-sdk` 后端则经由 [`subprocess/`](../subprocess/README.md) seam spawn 其子进程（共享的凭据清除、以进程树为范围的拆卸、dispose（资源释放）阶梯）。测试只用包内 fixture（测试前置数据）替换子 agent 边界。
 
 提案与设计理由见 [.agents/notes/implemented/feature/2026-06-21-subagent-capability-seam.md](../../.agents/notes/implemented/feature/2026-06-21-subagent-capability-seam.md)。