docs: add tip for setting default profile in configuration

[nebrass]

Summary

• Added a tip in the README explaining how to set a default profile in the configuration
• Users can now understand how to avoid typing --profile flag repeatedly

What

This PR adds documentation in the "Using Open Source Models" section that explains how to set a custom profile as the default by adding profile = "o3" at the top level of ~/.codex/config.toml.

Why

Users who frequently use custom profiles (like o3 or mistral) currently need to specify --profile with every command. This documentation helps them configure a default profile for a smoother workflow.

How

Added a tip box with clear instructions and example configuration showing how to set the profile parameter in the config file.

Testing

• Documentation-only change
• Verified markdown formatting is correct
• Example provided follows existing configuration patterns in the codebase

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[nebrass]

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

[etraut-openai]

Thanks for the contribution, and apologies for the slow response. We've received many PRs, and we don't have the bandwidth on the codex team to review all of them.

I think this PR is no longer relevant because the underlying documentation has change significantly since then. I'm going to close it out.