feat: introduce npm module for codex-responses-api-proxy (#4417)

This PR expands `.github/workflows/rust-release.yml` so that it also
builds and publishes the `npm` module for
`@openai/codex-responses-api-proxy` in addition to `@openai/codex`. Note
both `npm` modules are similar, in that they each contain a single `.js`
file that is a thin launcher around the appropriate native executable.
(Since we have a minimal dependency on Node.js, I also lowered the
minimum version from 20 to 16 and verified that works on my machine.)

As part of this change, we tighten up some of the docs around
`codex-responses-api-proxy` and ensure the details regarding protecting
the `OPENAI_API_KEY` in memory match the implementation.

To test the `npm` build process, I ran:

```
./codex-cli/scripts/build_npm_package.py --package codex-responses-api-proxy --version 0.43.0-alpha.3
```

which stages the `npm` module for `@openai/codex-responses-api-proxy` in
a temp directory, using the binary artifacts from
https://github.com/openai/codex/releases/tag/rust-v0.43.0-alpha.3.

codex-cli/package-lock.json

@@ -11,7 +11,7 @@
         "codex": "bin/codex.js"
       },
       "engines": {
-        "node": ">=20"
+        "node": ">=16"
       }
     }
   }