[codex] clarify Codex skill loading

[immy2good]

Summary

• Clarifies that Codex loads plugin skills natively.
• Points Codex agents at references/codex-tools.md for tool-name translation.
• Keeps Copilot and Gemini guidance intact.

Validation

• git diff --check
• Select-String focused checks for Codex loading/tool-mapping wording

Notes

• Opened as draft because this touches core skill instructions and should be reviewed against upstream contribution expectations before merge.