chore: add ValidationError usage rule to AGENTS.md

Author: tejaskash

AGENTS.md

@@ -103,3 +103,7 @@ bun --hot ./index.ts
 ```
 
 For more information, read the Bun API docs in `node_modules/bun-types/docs/**.mdx`.
+
+## Error Handling
+
+If a validation error is needed (user-correctable input problem), use `ValidationError` directly. Do not create subclasses like `MissingCodeDirectoryError extends ValidationError` — just throw `new ValidationError("code directory not found: ...")` with a descriptive message.