docs: add helper inlining rule

AGENTS.md

@@ -11,6 +11,7 @@
 
 - Read files in full before making wide-ranging changes, before editing files you have not already fully inspected, and when the user asks you to investigate or audit something. Do not rely only on search snippets for broad changes.
 - No `any` types unless absolutely necessary
+- Single-line helper functions with a single call site are forbidden; inline them instead.
 - Check node_modules for external API type definitions instead of guessing
 - **NEVER use inline imports** - no `await import("./foo.js")`, no `import("pkg").Type` in type positions, no dynamic imports for types. Always use standard top-level imports.
 - NEVER remove or downgrade code to fix type errors from outdated dependencies; upgrade the dependency instead