style: add code style instruction for switch statements

Author: ishulgin

AGENTS.md

@@ -12,6 +12,8 @@
 
 - Keep edits consistent with existing formatting.
 - When adding env/config knobs, document them in `readme-dev.md`.
+- When updating discriminated-union/event `switch` statements, do not add a trailing fallback like `return null` only to satisfy TypeScript.
+- Handle each variant with an explicit `case`; if intentionally ignored, use an explicit no-op case.
 
 ## Testing Guidelines