meta(agents): Update AI commit attribution guidance (#21166)

Clarify that AI commits should use the committer email associated with
the agent instead of assuming Anthropic-only attribution. Document known
OpenAI and Cursor committer emails, and allow omitting the attribution
line only when no committer email is known.

This will prevent stuff like this from happening

<img width="814" height="127" alt="image"
src="https://github.com/user-attachments/assets/286705c1-5810-46c1-8ef2-063605655e01"
/>

Co-authored-by: GPT-5 Codex <codex@openai.com>