Evaluate AGENTS.md and skills with Codex

[RoyLee1224]

cc: @jason810496

Follow up on #69308 by adding Codex as an alternative runtime for evaluating AGENTS.md and skills through promptfoo's built-in Codex SDK provider.

Run the evaluation with Codex:

AGENT_RUNTIME=codex \
  prek run run-skill-eval --hook-stage manual --all-files

The evaluation completed all six cases with no provider errors. Four behavioral assertions passed and two existing ambiguous newsfragment cases failed.

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Was generative AI tooling used to co-author this PR?

• Yes gpt-5.6-sol

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• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.