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.

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/757).
* #763
* __->__ #757

codex-cli/package.json

@@ -20,10 +20,7 @@
     "typecheck": "tsc --noEmit",
     "build": "node build.mjs",
     "build:dev": "NODE_ENV=development node build.mjs --dev && NODE_OPTIONS=--enable-source-maps node dist/cli-dev.js",
-    "release:readme": "cp ../README.md ./README.md",
-    "release:version": "TS=$(date +%y%m%d%H%M) && sed -E -i'' -e \"s/\\\"0\\.1\\.[0-9]{10}\\\"/\\\"0.1.${TS}\\\"/g\" package.json",
-    "release:build-and-publish": "pnpm run build && npm publish",
-    "release": "pnpm run release:readme && pnpm run release:version && pnpm install && pnpm run release:build-and-publish"
+    "stage-release": "./scripts/stage_release.sh"
   },
   "files": [
     "dist"