Load plugin manifests through a shared discoverable-path helper so
manifest reads, installs, and skill names all see the same alternate
manifest location.
## Summary
- move skill loading and management into codex-core-skills
- leave codex-core with the thin integration layer and shared wiring
## Testing
- CI
---------
Co-authored-by: Codex <noreply@openai.com>
## Summary
- extract plugin identifiers and load-outcome types into codex-plugin
- update codex-core to consume the new plugin crate
## Testing
- CI
---------
Co-authored-by: Codex <noreply@openai.com>
