Fix Windows npm install regression from dist-tag packaging change

The recent packaging migration in #11339 switched platform artifacts to be published as `@openai/codex` with platform-specific versions and wired the meta package via npm alias optional dependencies (for example `@openai/codex-win32-x64 -> npm:@openai/codex@0.101.0-win32-x64`).

This caused runtime failures on Windows where `codex` reports:
`Missing optional dependency @openai/codex-win32-x64`.

### What changed
- Updated `codex-cli/scripts/build_npm_package.py` to publish platform artifacts under their concrete package names:
  - `@openai/codex-win32-x64`
  - `@openai/codex-win32-arm64`
  - `@openai/codex-linux-x64`
  - `@openai/codex-linux-arm64`
  - `@openai/codex-darwin-x64`
  - `@openai/codex-darwin-arm64`
- Updated meta package `optionalDependencies` to use plain version references (e.g. `0.101.0-win32-x64`) instead of npm alias syntax to `@openai/codex`.
- Updated `codex-cli/scripts/README.md` to reflect that platform-native artifacts are separate optional dependency packages.

### Why
`bin/codex.js` resolves platform packages by name (`@openai/codex-win32-x64`, etc). Publishing platform tarballs under `@openai/codex` introduces alias indirection that can fail to materialize these package names at install time, especially on Windows. Publishing under the concrete names restores direct name alignment and reliable resolution.

codex-cli/scripts/README.md

@@ -15,8 +15,8 @@ This downloads the native artifacts once, hydrates `vendor/` for each package, a
 tarballs to `dist/npm/`.
 
 When `--package codex` is provided, the staging helper builds the lightweight
-`@openai/codex` meta package plus all platform-native `@openai/codex` variants
-that are later published under platform-specific dist-tags.
+`@openai/codex` meta package plus all platform-native packages (for example
+`@openai/codex-win32-x64`) that are consumed as optional dependencies.
 
 If you need to invoke `build_npm_package.py` directly, run
 `codex-cli/scripts/install_native_deps.py` first and pass `--vendor-src` pointing to the

codex-cli/scripts/build_npm_package.py

@@ -16,8 +16,8 @@ RESPONSES_API_PROXY_NPM_ROOT = REPO_ROOT / "codex-rs" / "responses-api-proxy" /
 CODEX_SDK_ROOT = REPO_ROOT / "sdk" / "typescript"
 CODEX_NPM_NAME = "@openai/codex"
 
-# `npm_name` is the local optional-dependency alias consumed by `bin/codex.js`.
-# The underlying package published to npm is always `@openai/codex`.
+# `npm_name` is the platform package consumed by `bin/codex.js` as an
+# optional dependency of the meta package.
 CODEX_PLATFORM_PACKAGES: dict[str, dict[str, str]] = {
     "codex-linux-x64": {
         "npm_name": "@openai/codex-linux-x64",
@@ -263,7 +263,7 @@ def stage_sources(staging_dir: Path, version: str, package: str) -> None:
             codex_package_json = json.load(fh)
 
         package_json = {
-            "name": CODEX_NPM_NAME,
+            "name": platform_package["npm_name"],
             "version": platform_version,
             "license": codex_package_json.get("license", "Apache-2.0"),
             "os": [platform_package["os"]],
@@ -304,9 +304,8 @@ def stage_sources(staging_dir: Path, version: str, package: str) -> None:
     if package == "codex":
         package_json["files"] = ["bin"]
         package_json["optionalDependencies"] = {
-            CODEX_PLATFORM_PACKAGES[platform_package]["npm_name"]: (
-                f"npm:{CODEX_NPM_NAME}@"
-                f"{compute_platform_package_version(version, CODEX_PLATFORM_PACKAGES[platform_package]['npm_tag'])}"
+            CODEX_PLATFORM_PACKAGES[platform_package]["npm_name"]: compute_platform_package_version(
+                version, CODEX_PLATFORM_PACKAGES[platform_package]["npm_tag"]
             )
             for platform_package in PACKAGE_EXPANSIONS["codex"]
             if platform_package != "codex"