Add guidelines for Rust-backed Python classes and add Gemini workspace context document

- Updated AGENTS.md to include instructions for handling Python classes that wrap Rust implementations: update Rust code first, then Python wrappers.
- Added GEMINI.md with detailed project overview, key technologies, repo structure, build instructions, testing, and linting info to assist in code assistance tasks.

Author: kosiew

AGENTS.md

@@ -47,6 +47,12 @@ When generating Rust code for DataFusion, follow these guidelines:
 
 3. Follow the standard Rust style conventions from rustfmt
 
+## Rust-backed Classes
+
+Some Python classes in this project are thin wrappers around Rust
+implementations. If a change affects such a class, update the Rust code
+first and then adjust the Python wrappers accordingly.
+
 ## Code Organization
 - Keep functions focused and under about 50 lines.
 - Break complex tasks into well-named helper functions and reuse existing