ai(rules[AGENTS]) Clarify namespace imports for stdlib only

Author: tony

AGENTS.md

@@ -190,7 +190,7 @@ def test_sync(
 
 ### Imports
 
-- Use namespace imports: `import enum` instead of `from enum import Enum`
+- Use namespace imports for stdlib: `import enum` instead of `from enum import Enum`; third-party packages may use `from X import Y`
 - For typing, use `import typing as t` and access via namespace: `t.NamedTuple`, etc.
 - Use `from __future__ import annotations` at the top of all Python files