feat(AGENTS.md): refine the ai-dev-guide into standard AGENTS.md

Author: keli-wen

ai-dev-guide.md AGENTS.mdai-dev-guide.md renamed to AGENTS.md

@@ -28,3 +28,10 @@ llm_block = self._llm_blocks[identifier]  # KeyError naturally thrown
 - **Side effects**: Logging, state changes, external calls
 
 **Remember:** Encapsulation should hide complexity, not add complexity.
+
+## Impl with Unit Test and Example
+
+If you are implementing a new feature, please implement the unit test and example.
+
+- For unit test, add in `tests/<module_name>`, and inherit the `unittest.TestCase` class.
+- For example, add in `examples/<module_name>`, and just demo the simple usage. (do not add too many use cases in single file)