Finalize `approval_policy` configuration option

Currently, the accepted values for the [`approval_policy`](https://github.com/openai/codex/blob/main/codex-rs/config.md#approval_policy) config option are:

- `unless-allow-listed`
- `on-failure`
- `never`

It would be nice to come up with names that were more pithy/obvious.