[codex] add coauthor conventions

[hartsock]

What changed

• Added AGENTS.md for non-Claude agents.
• Added CLAUDE.md for Claude sessions.
• Copied the newt-agent model attribution convention for Co-authored-by trailers.

Why

This gives gitxtend the same commit provenance convention used in newt-agent, so LLM contributions are attributed consistently in commit messages.

Validation

• git diff --cached --check
• git diff --check HEAD~1..HEAD
• cargo fmt --all --check