feat: include Windows binary of the CLI in the npm release (#2040)

To date, the build scripts in `codex-cli` still supported building the
old TypeScript version of the Codex CLI to give Windows users something
they can run, but we are just going to have them use the Rust version
like everyone else, so:

- updates `codex-cli/bin/codex.js` so that we run the native binary or
throw if the target platform/arch is not supported (no more conditional
usage based on `CODEX_RUST`, `use-native` file, etc.)
- drops the `--native` flag from `codex-cli/scripts/stage_release.sh`
and updates all the code paths to behave as if `--native` were passed
(i.e., it is the only way to run it now)

Tested this by running:

```
./codex-cli/scripts/stage_rust_release.py --release-version 0.20.0-alpha.2
```