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/output/detail.rs

@@ -211,13 +211,15 @@ fn install_details(parsed: &[ParsedDetail], options: HumanOutputOptions) -> Vec<
 
     let managed_by_npm = value(parsed, "managed by npm").unwrap_or("false");
     let managed_by_bun = value(parsed, "managed by bun").unwrap_or("false");
+    let managed_by_pnpm = value(parsed, "managed by pnpm").unwrap_or("false");
     let package_root = value(parsed, "managed package root").unwrap_or("not set");
     out.push(HumanDetail::Row {
         label: "managed by".to_string(),
         value: format!(
-            "npm: {} · bun: {} · package root {}",
+            "npm: {} · bun: {} · pnpm: {} · package root {}",
             yes_no(managed_by_npm),
             yes_no(managed_by_bun),
+            yes_no(managed_by_pnpm),
             if is_falsy(package_root) {
                 "—".to_string()
             } else {
@@ -263,6 +265,7 @@ fn install_details(parsed: &[ParsedDetail], options: HumanOutputOptions) -> Vec<
             "install context",
             "managed by npm",
             "managed by bun",
+            "managed by pnpm",
             "managed package root",
             "PATH codex entries",
         ],