Add JJ version control requirement to AGENTS.md

AGENTS.md

@@ -154,6 +154,10 @@ mod tests {
 - Write clear, descriptive commit messages
 - Ensure the workspace compiles: `cargo build --workspace`
 
+### Version Control
+- **Use JJ (Jujutsu)** as the primary tool for all source control operations due to its concurrency and conflict-free design
+- **Git fallback**: Only for complex operations unsupported by JJ (e.g., interactive rebasing)
+
 ## Project Structure Guidelines
 
 ### Core Module (`banks2ff/src/core/`)