`codex exec` option to skip the inner sandbox for already-isolated runtimes (external-sandbox / BYO-seccomp)

What variant of Codex are you using?

CLI

What feature would you like to see?

Please expose an "external sandbox / no inner sandbox" option on codex exec (a flag or sandbox_mode = "external") for environments that are already isolated (Lambda, gVisor, hardened containers). Same environment as #4725 (closed with a fork/EC2 workaround). apply_patch works; only child-process spawning is blocked.

Additional information

codex exec --dangerously-bypass-approvals-and-sandbox (sandbox=danger-full-access) still fails to spawn shell processes inside AWS Lambda with Operation not permitted (EPERM): Codex installs a LinuxSeccomp jail on every shell child (danger-full-access → PermissionProfile::Disabled → unrestricted() → should_require_platform_sandbox = true → SandboxType::LinuxSeccomp), and Lambda's own seccomp profile denies the nested install. The only profile yielding SandboxType::None is ExternalSandbox, which the exec CLI/config can't select (SandboxMode = read-only|workspace-write|danger-full-access).

Start with the exec CLI/config handling for SandboxMode and trace how PermissionProfile selects SandboxType for shell children. Verify the new external-sandbox option produces SandboxType::None and allows child-process spawning in an already-isolated runtime such as AWS Lambda without installing a nested LinuxSeccomp jail.