docs: stop Codex false-alarm on DCO sign-off (#8561)
## What
Adds one review-scope rule to `AGENTS.md`/`CLAUDE.md`: commit trailers
are not visible in a diff, so reviewers must verify `Signed-off-by` via
`git log`/commits API before flagging it as missing.
## Why
Codex (`chatgpt-codex-connector[bot]`) alarms "Add the required DCO
sign-off" on PRs whose commits are already signed off (#8543, #8537).
Root cause: its review context is diff-only and includes only commit
titles
([review_request.rs](https://github.com/openai/codex/blob/main/codex-rs/prompts/src/review_request.rs)),
while the [review
rubric](https://github.com/openai/codex/blob/main/codex-rs/prompts/templates/review/rubric.md)
mandates rule attribution against AGENTS.md — so the model asserts a
missing trailer it cannot see. The rubric states user instructions about
review scope take precedence, so this rule suppresses the false alarm at
the source while keeping the sign-off requirement for commit creation.
Full analysis: #8558 (comment).
Fixes #8558
Signed-off-by: Ma, Guokai <guokai.ma@intel.com>
