Document PLANS.md

[friel-openai]

Summary

Documents the topic of one of our DevDay 2025 talks, using a PLANS.md to enable coding agents to work on larger, more complex tasks and stay on task for hours of work.

Motivation

We shared with our audience that this would be published to improve long task behavior with the gpt-5-codex model, Codex CLI and more.

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[friel-openai]

@codex review

[kiralpoon]

Thank you so much for update this right away after the session!! This would be super helpful!! ☺️

[rezkam]

Thanks for sharing these files! @friel-openai
Just curious, would something like github/spec-kit work for this? I noticed some custom terms like ExccPlans that seem fairly comparable to what spec-kit provides, and it also supports codex.