clarify test review wording

AGENTS.md

@@ -108,6 +108,15 @@ See `codex-rs/tui/styles.md`.
 
 ## Tests
 
+When reviewing or adding tests:
+
+- Ask yourself whether the test makes sense.
+- Ask yourself whether it actually asserts something valuable that you care about.
+- Ask yourself how much the test would need to change if the surrounding system were refactored.
+- Ask yourself whether you or someone else would notice the regression if Codex auto-updated the test.
+
+If a unit test mostly covers deep implementation details, static data, removed behavior, or requires heavy monkeypatching, prefer deleting it and writing an integration test that protects meaningful end-to-end behavior instead.
+
 ### Snapshot tests
 
 This repo uses snapshot tests (via `insta`), especially in `codex-rs/tui`, to validate rendered output.