feat: add effort option (similar to #6)

[artdgn]

Adding an option to specify effort for better results / reviews.

Details:

• I followed the changes in add model option #6.
• Since there's no dedicated flag for effort, I pass it as inline config value, following the instructions in https://github.com/openai/codex/blob/main/docs/config.md
• Haven't seen any test targets so haven't tested, but did run build and check and they are fine.

Testing

• Tested the forked action (artdgn/[email protected]) when specifying effort on a different repo and it works.

[github-actions]

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[artdgn]

I have read the CLA Document and I hereby sign the CLA