Detect Codex installs managed by pnpm (#31503)

## Why

The Codex JavaScript shim currently distinguishes npm and Bun installs,
but a global pnpm install falls back to npm. That causes the native CLI,
`codex doctor`, and update flows to report or run npm commands even
though pnpm owns the installation. pnpm also allows its global package
and bin directories to differ, so `PNPM_HOME` does not reliably identify
the package owner.

## What changed

- detect pnpm-managed installs by finding pnpm's
`node_modules/.modules.yaml` metadata from the launched JavaScript
entrypoint
- pass a mutually exclusive `CODEX_MANAGED_BY_PNPM` marker into the
native CLI
- represent pnpm in install context, doctor output, version checks, and
TUI update actions without changing the existing public
`InstallContext::from_exe` signature
- recommend `pnpm add -g @openai/codex` for pnpm-managed installs and
snapshot the rendered TUI update notice

Validated with the 9-test install-context suite, the focused pnpm TUI
snapshot test, and Node's syntax check for the launcher.

Closes https://github.com/openai/codex/issues/10294.

codex-rs/cli/src/doctor.rs

@@ -797,6 +797,10 @@ fn installation_check(show_details: bool) -> DoctorCheck {
         "managed by bun: {}",
         env::var_os("CODEX_MANAGED_BY_BUN").is_some()
     ));
+    details.push(format!(
+        "managed by pnpm: {}",
+        env::var_os("CODEX_MANAGED_BY_PNPM").is_some()
+    ));
     push_env_path_detail(
         &mut details,
         "managed package root",
@@ -885,6 +889,7 @@ fn doctor_managed_by_npm(current_exe: Option<&Path>) -> bool {
 fn inherited_managed_env_for_cargo_binary(current_exe: Option<&Path>) -> bool {
     if env::var_os("CODEX_MANAGED_BY_NPM").is_none()
         && env::var_os("CODEX_MANAGED_BY_BUN").is_none()
+        && env::var_os("CODEX_MANAGED_BY_PNPM").is_none()
     {
         return false;
     }
@@ -937,6 +942,9 @@ fn describe_install_context(context: &InstallContext) -> String {
         InstallMethod::Bun => {
             describe_method_with_package_layout("bun", context.package_layout.as_ref())
         }
+        InstallMethod::Pnpm => {
+            describe_method_with_package_layout("pnpm", context.package_layout.as_ref())
+        }
         InstallMethod::Brew => {
             describe_method_with_package_layout("brew", context.package_layout.as_ref())
         }