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

Author: tony

AGENTS.md

@@ -210,8 +210,9 @@ Key highlights:
 
 ### Imports
 
-- **Use namespace imports**: `import enum` instead of `from enum import Enum`
+- **Use namespace imports for standard library modules**: `import enum` instead of `from enum import Enum`
   - **Exception**: `dataclasses` module may use `from dataclasses import dataclass, field` for cleaner decorator syntax
+  - This rule applies to Python standard library only; 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