Drop stale update-readme branch reference from AGENTS.md

That branch has been deleted (local and remote), so the warning about
it no longer applies.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FXcrACM5sok3M1KQJ8kByy

AGENTS.md

@@ -15,9 +15,6 @@ a manually typed-in rate).
 
 This is a from-scratch rewrite of an earlier Skip-based (Swift-transpiled-to-
 Kotlin) implementation — `main` is now this Kotlin Multiplatform project.
-Any branch still describing a `skip build`/Swift Package Manager workflow
-(e.g. `update-readme`) predates the rewrite and isn't a useful reference for
-anything in this codebase.
 
 ## Module layout