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/bin/codex.js

@@ -1,6 +1,7 @@
 #!/usr/bin/env node
 // Unified entry point for the Codex CLI.
 
+import { spawn } from "node:child_process";
 import { existsSync } from "fs";
 import path from "path";
 import { fileURLToPath } from "url";
@@ -68,7 +69,6 @@ const binaryPath = path.join(archRoot, "codex", codexBinaryName);
 // executing. This allows us to forward those signals to the child process
 // and guarantees that when either the child terminates or the parent
 // receives a fatal signal, both processes exit in a predictable manner.
-const { spawn } = await import("child_process");
 
 function getUpdatedPath(newDirs) {
   const pathSep = process.platform === "win32" ? ";" : ":";