Reject cyclic reply headers before walking email thread ancestry

[rudycelekli]

Agent Information

Agent file: engineering/engineering-email-intelligence-engineer.md

Motivation

Two messages replying to each other or a self-reply produce cyclic parent graphs; parent-body walkers can loop forever.

Testing

• Reproduced the original example failure; the corrected example passes the same focused fixture.
• Changed-agent lint and git diff --check pass.
• Full scripts/test-convert-outputs.sh --drift=advisory passes all 32 checks over 282 agents and 15 tool formats. Expected content drift is advisory; the generated-output manifest is left to maintainers.

Native extracted example with controlled local inputs; no production system or model-effectiveness claim. No paid API call, real invoice, production data or actual model-effectiveness evaluation was performed.

Candidate listed before publication in #917: #917 (comment). This single-profile correction adds no tooling or generated outputs. AI assistance was used to investigate, implement and test it; the commit is signed and includes DCO sign-off.