chore: introduce publishing logic for @openai/codex-sdk (#4543)

There was a bit of copypasta I put up with when were publishing two
packages to npm, but now that it's three, I created some more scripts to
consolidate things.

With this change, I ran:

```shell
./scripts/stage_npm_packages.py --release-version 0.43.0-alpha.8 --package codex --package codex-responses-api-proxy --package codex-sdk
```

Indeed when it finished, I ended up with:

```shell
$ tree dist
dist
└── npm
    ├── codex-npm-0.43.0-alpha.8.tgz
    ├── codex-responses-api-proxy-npm-0.43.0-alpha.8.tgz
    └── codex-sdk-npm-0.43.0-alpha.8.tgz
$ tar tzvf dist/npm/codex-sdk-npm-0.43.0-alpha.8.tgz
-rwxr-xr-x  0 0      0    25476720 Oct 26  1985 package/vendor/aarch64-apple-darwin/codex/codex
-rwxr-xr-x  0 0      0    29871400 Oct 26  1985 package/vendor/aarch64-unknown-linux-musl/codex/codex
-rwxr-xr-x  0 0      0    28368096 Oct 26  1985 package/vendor/x86_64-apple-darwin/codex/codex
-rwxr-xr-x  0 0      0    36029472 Oct 26  1985 package/vendor/x86_64-unknown-linux-musl/codex/codex
-rw-r--r--  0 0      0       10926 Oct 26  1985 package/LICENSE
-rw-r--r--  0 0      0    30187520 Oct 26  1985 package/vendor/aarch64-pc-windows-msvc/codex/codex.exe
-rw-r--r--  0 0      0    35277824 Oct 26  1985 package/vendor/x86_64-pc-windows-msvc/codex/codex.exe
-rw-r--r--  0 0      0        4842 Oct 26  1985 package/dist/index.js
-rw-r--r--  0 0      0        1347 Oct 26  1985 package/package.json
-rw-r--r--  0 0      0        9867 Oct 26  1985 package/dist/index.js.map
-rw-r--r--  0 0      0          12 Oct 26  1985 package/README.md
-rw-r--r--  0 0      0        4287 Oct 26  1985 package/dist/index.d.ts
```

codex-cli/scripts/README.md

@@ -1,11 +1,19 @@
 # npm releases
 
-Run the following:
-
-To build the 0.2.x or later version of the npm module, which runs the Rust version of the CLI, build it as follows:
+Use the staging helper in the repo root to generate npm tarballs for a release. For
+example, to stage the CLI, responses proxy, and SDK packages for version `0.6.0`:
 
 ```bash
-./codex-cli/scripts/build_npm_package.py --release-version 0.6.0
+./scripts/stage_npm_packages.py \
+  --release-version 0.6.0 \
+  --package codex \
+  --package codex-responses-api-proxy \
+  --package codex-sdk
 ```
 
-Note this will create `./codex-cli/vendor/` as a side-effect.
+This downloads the native artifacts once, hydrates `vendor/` for each package, and writes
+tarballs to `dist/npm/`.
+
+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
+directory that contains the populated `vendor/` tree.