chore: make build process a single script to run (#757)

This introduces `./codex-cli/scripts/stage_release.sh`, which is a shell
script that stages a release for the Node.js module in a temp directory.
It updates the release to include these native binaries:

```
bin/codex-linux-sandbox-arm64
bin/codex-linux-sandbox-x64
```

though this PR does not update Codex CLI to use them yet.

When doing local development, run
`./codex-cli/scripts/install_native_deps.sh` to install these in your
own `bin/` folder.

This PR also updates `README.md` to document the new workflow.

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[//]: # (BEGIN SAPLING FOOTER)
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* #763
* __->__ #757

codex-cli/.gitignore

@@ -0,0 +1,3 @@
+# Added by ./scripts/install_native_deps.sh
+/bin/codex-linux-sandbox-arm64
+/bin/codex-linux-sandbox-x64