Add macOS and Linux direct install script (#12740)

## Summary
- add a direct install script for macOS and Linux at
`scripts/install/install.sh`
- stage `install.sh` into `dist/` during release so it is published as a
GitHub release asset
- reuse the existing platform npm payload so the installer includes both
`codex` and `rg`

## Testing
- `bash -n scripts/install/install.sh`
- local macOS `curl | sh` smoke test against a locally served copy of
the script

.github/workflows/rust-release.yml

@@ -494,6 +494,10 @@ jobs:
             --package codex-responses-api-proxy \
             --package codex-sdk
 
+      - name: Stage macOS and Linux installer script
+        run: |
+          cp scripts/install/install.sh dist/install.sh
+
       - name: Create GitHub Release
         uses: softprops/action-gh-release@v2
         with: