feat: add codex proxy sidecar for NOFX

[roberto-ryan]

Summary

• add a dedicated chatgpt-codex-proxy Go service to proxy Codex/OpenAI-compatible requests for NOFX
• wire the sidecar into Docker Compose and nginx so the app can reach the proxy in deployment
• update model handling and URL validation logic to support the new proxy path safely
• add tests and operational docs for the proxy service

Test Plan

• Go unit tests added for the new proxy service and URL validation updates
• Full integration/deployment validation in the target environment

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[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

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