revert(sandbox): withdraw the credential-document read denial

The `readDenyPaths` policy field shipped in the previous commit broke Linux
confinement outright. bwrap has to create the `/dev/null` bind's mount point
inside a tree its own profile has already made read-only, so it refused the
entire confinement whenever the parent directory was absent — every host that
has not stored a credential yet, including a fresh install:

  bwrap: Can't mkdir parents for /home/runner/.dsh/.env: Read-only file system

which the executor correctly classifies as SANDBOX_UNAVAILABLE, so every
confined bash call failed closed. Landlock cannot subtract from its own `/`
read grant, so it reported `partial` enforcement on every confined call for a
file it never hid, with no way to switch the denial off (schemastery fills an
omitted array with `[]`, so empty and omitted were indistinguishable).

A protection that breaks confinement where it works and misreports it where it
does not is worse than a documented absence. Revert the field, both expressible
backends, the enforcement downgrade, and the policy default; state the residue
plainly in the credentials-local READMEs — file mode stops other OS users, not
the model — and keep the OS-keychain provider recorded as the real answer.

The narrower discipline stands: no surface hoists the credential document into
`process.env`, and the model is never handed a resolved path to it.

packages/credentials/credentials-local/README.zh.md

@@ -32,12 +32,9 @@ dotenv 格式，用 `dotenv` 解析；写回用物理行级编辑器，保留一
 
 ## 安全边界
 
-文档在 `0700` 目录下以 `0600` 权限存放，这挡得住其他 OS 用户，**挡不住**模型。工具进程（bash、文件系统工具）以同一用户身份运行，因此在出厂默认的 `danger-full-access` 下，它们读这个文件与读该用户拥有的任何其他文件毫无二致。有两件事收窄了这一点：
+文档在 `0700` 目录下以 `0600` 权限存放，这挡得住其他 OS 用户，**挡不住**模型。工具进程（bash、文件系统工具）以同一用户身份运行，因此在出厂默认的 `danger-full-access` 下，它们读这个文件与读该用户拥有的任何其他文件毫无二致，也没有任何沙箱模式会把它单独挑出来。harness 真正守住的更窄：它绝不把该文档的解析后路径交给模型，也绝不把它载入进程环境（见 [app-boot 的个人配置](../../ui/app-boot/README.md#personal-config)），因此要拿到这个值，需要刻意去读一条并未交给 agent 的路径。
 
-- **受限沙箱模式**会专门拒绝凭据文档：[`dsh-sandbox-policy`](../../sandbox/sandbox-policy/README.md) 把 `readDenyPaths` 默认为 `$DSH_HOME/.env`，Seatbelt 与 bwrap 后端会执行它（Landlock 无法从自己的 `/` 读授权中扣除，只能报 `partial`）。这条拒绝点名的是该文件而非整个 home，因此模型对自己会话日志的既定访问不受影响。
-- harness 绝不把该文档的解析后路径交给模型，也绝不把它载入进程环境（见 [app-boot 的个人配置](../../ui/app-boot/README.md#personal-config)）。
-
-这两者都不能让未受限的 agent 变得安全。必须让提供方密钥远离自身 agent 的部署应当运行受限模式；OS 钥匙串 provider——一个模型的进程根本读不到的存储——才是延后的答案，它应当作为平级包与本 provider 并列。
+这是审慎，不是边界。必须让提供方密钥远离自身 agent 的部署无法靠文件权限做到；OS 钥匙串 provider——一个模型的进程根本读不到的存储——才是延后的答案，它应当作为平级包与本 provider 并列。
 
 ## Model Experience