Clarify Codex commit co-author attribution behavior

What is the issue?

I was trying to understand whether Codex should add a Co-authored-by trailer when it creates commits, similar to Claude Code / Aider conventions.

Current behavior is unclear:

• By default, should Codex-created commits include a Co-authored-by: Codex ... trailer?
• Is commit_attribution expected to take effect only when [features] codex_git_commit = true is enabled?
• If attribution is enabled, what exact config value is supported and what default identity is used?
• Should this be opt-in, opt-out, or documented as unsupported for normal user-driven git commit commands?

Related:

• #14051
• #14994
• #18095
• #938

Why this matters

Users want clear provenance for AI-assisted commits without accidentally misattributing authorship or adding an incorrect trailer. Clear docs or a supported config path would make this less ambiguous.