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

.github/workflows/ci.yml

@@ -68,6 +68,12 @@ jobs:
       - name: Build
         run: pnpm run build
 
+      - name: Ensure staging a release works.
+        working-directory: codex-cli
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: pnpm stage-release
+
       - name: Ensure README.md contains only ASCII and certain Unicode code points
         run: ./scripts/asciicheck.py README.md
       - name: Check README ToC