docs: add refactoring opportunities to avoid private API usage in AGENTS.md

Author: kosiew

AGENTS.md

@@ -23,6 +23,25 @@ This repository contains Python bindings for Rust's DataFusion.
 - The repository mixes Python and Rust; ensure changes build for both languages.
 - If adding new dependencies, update `pyproject.toml` and run `uv sync --dev --no-install-package datafusion`.
 
+## Refactoring opportunities
+  - Avoid using private or low-level APIs when a stable, public helper exists. For example,
+    automated refactors should spot and replace uses: 
+
+    ```python
+    # Before (uses private/low-level API) 
+    # PyArrow example
+    reader = pa.RecordBatchReader._import_from_c_capsule(
+            df.__arrow_c_stream__()
+    )
+
+    # After (use public API)
+    reader = pa.RecordBatchReader.from_stream(df)
+    ```
+
+    Look for call chains that invoke `_import_from_c_capsule` with `__arrow_c_stream__()`
+    and prefer `from_stream(df)` instead. This improves readability and avoids
+    relying on private PyArrow internals that may change.
+
 ## Helper Functions
 - `python/datafusion/io.py` offers global context readers:
   - `read_parquet`