Add Codex review guidelines for PR intent

AGENTS.md

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+# Agent Instructions
+
+## Review guidelines
+
+Respect documented author intent and check for companion PRs.
+
+- Read the PR body before the diff. If it has sections like "Design
+  decisions", "Companion PRs", "Cross-repo", or "Intent", treat them as
+  the author's binding statement of design — constraints, not code to
+  second-guess. Do not raise a finding that would contradict a documented
+  choice.
+- Assume work often spans multiple repos in this org. "Missing caller",
+  "dead code", and "unused export" findings are often wrong because the
+  caller lives in a companion PR. Before flagging dead or missing code,
+  check whether the PR description references companion PRs in other
+  repos.
+- When author intent is unclear, ask a question rather than request a
+  change.