use codex native installer

[BradSwain]

The native installer script should automatically handle installing any utilities codex wants to bundle. The native installer also allows codex upgrade to work from within a vm.

[hbrodin]

Looks good to me. Using the native installer makes sense for keeping the bundled tools and self-update behavior aligned with upstream.

One nonblocking question: does upstream provide a supported way to clean up older completed releases? The installer currently retains them, whereas coop's previous installer pruned inactive releases. If there isn't an upstream cleanup mechanism, I'm happy to track this as a follow-up rather than couple coop to the native package layout here.

Review coverage: correctness, design, conventions, security, API usage, tests, docs, and comments. No blocking findings. All nine host probe tests passed, four deliberately broken variants were caught, and shell syntax checks passed. CI is green; Lima and Firecracker VM integration were not run in this review and are absent from the PR checks.

[BradSwain]

does upstream provide a supported way to clean up older completed releases?

Doesn't look like the installer supports cleaning up older releases.

I can add a follow-on PR to have coop handle cleanup. I believe the installer is part of the open source codex repo, so we could potentially open a PR updating the installer itself as well.