[codex] add coauthor conventions

AGENTS.md

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+# gitxtend - Agent Instructions (general)
+
+Mirror of `CLAUDE.md` for non-Claude agents (Codex, Gemini, local
+models, etc.). When the two files disagree, `CLAUDE.md` is canonical for
+Claude sessions and this file is canonical for everyone else.
+
+## Model attribution
+
+- If an LLM materially contributes to a commit, identify it with a
+  `Co-authored-by` trailer in the commit message.
+- Use the model/tool identity the session is actually running under. Do not
+  credit a generic "AI Assistant".
+- Known trailers:
+  - `Co-authored-by: Codex <codex@openai.com>`
+  - `Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>`
+- If multiple LLMs contribute to the same commit, include one trailer per
+  contributing model.

CLAUDE.md

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+# gitxtend - Agent Instructions (Claude)
+
+This file is loaded by Claude Code on every session in this repository.
+Read it once at session start; the constraints below apply for the rest
+of the session unless there is explicit human authorization to deviate.
+
+## Model attribution
+
+- If an LLM materially contributes to a commit, identify it with a
+  `Co-authored-by` trailer in the commit message.
+- Use the model/tool identity the session is actually running under. Do not
+  credit a generic "AI Assistant".
+- Known trailers:
+  - `Co-authored-by: Codex <codex@openai.com>`
+  - `Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>`
+- If multiple LLMs contribute to the same commit, include one trailer per
+  contributing model.