chore: add empty Unreleased section above 6.1.0 in CHANGELOG

Keeps the standard Keep a Changelog workflow with an Unreleased section
at the top for tracking future changes post-release.

Co-authored-by: Greg Nazario <greg@gnazar.io>

Author: cursoragent

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 All notable changes to the Aptos TypeScript SDK will be captured in this file. This changelog is written by hand for now. It adheres to the format set out by [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
+# Unreleased
+
 # 6.1.0 (2026-02-25)
 
 ## Added