feat: hex.pm publish prep

## Summary

Prepare for hex.pm publication:
- Package config in mix.exs (description, licenses, links, source_url)
- Rename to codex_wrapper (drop the _ex suffix) if available on hex
- Verify: mix hex.build, mix docs
- Add @moduledoc and @doc to all public functions
- Typespec audit — all public functions should have @spec