[ai] Add instructions for concise comments (#18522)

AGENTS.md

@@ -763,3 +763,13 @@ The project uses English for non-code content. When drafting documentation, code
 PR descriptions, and similar text, avoid technical jargon. Instead, express concepts in plain English,
 using standard technical terms only when required. Ensure the text is readily comprehensible to a wide
 audience, including non-native English speakers.
+
+## 10. Code Comments
+
+Code comments on individual lines should be used only where necessary to flag issues that may not be obvious
+on a simple reading of the code. Keep them short (e.g. 1 or 2 lines).
+
+Function and method comment blocks may include more detail as required to make
+calling contracts clear and document parameter usage, but should still be kept concise.
+
+Avoid redundancy and repetition; comments should never simply restate what the code already says.