docs(CHANGES): Add guard comments to protect placeholder text

Prevent LLMs from accidentally removing the placeholder line
when adding changelog entries

Author: tony

CHANGES

@@ -16,6 +16,10 @@ $ uv add libvcs --prerelease allow
 
 ## libvcs 0.39.x (unreleased)
 
+<!-- KEEP THIS PLACEHOLDER - DO NOT REMOVE OR MODIFY THIS LINE -->
+_Notes on the upcoming release will go here._
+<!-- END PLACEHOLDER - ADD NEW CHANGELOG ENTRIES BELOW THIS LINE -->
+
 <!-- Maintainers, insert changes / features for the next release here -->
 
 ## libvcs 0.38.2 (2026-01-24)