Load plugin manifests through a shared discoverable-path helper so manifest reads, installs, and skill names all see the same alternate manifest location.
Migrates skill loading to support reading repo skills from the remote environment.
## Summary - extract shared plugin path and manifest helpers into codex-utils-plugins - update codex-core to consume the utility crate ## Testing - CI --------- Co-authored-by: Codex <noreply@openai.com>