Document strict Clippy validation

AGENTS.md

@@ -6,6 +6,21 @@ changes.
 
 ## Validation
 
+### Clippy
+
+All targets, including the library and its unit tests, must pass the crate's
+strict lint configuration with warnings denied:
+
+```sh
+cargo clippy --all-targets --all-features -- -D warnings
+```
+
+Do not consider a change complete while this command reports diagnostics.
+Prefer fixing diagnostics. When a lint intentionally conflicts with documented
+behavior, use the narrowest possible lint expectation and include a reason.
+
+### Ordinary tests
+
 Run the ordinary test suite with:
 
 ```sh