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

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

Author: tony

CHANGES

@@ -29,7 +29,9 @@ $ uvx --from 'vcspull' --prerelease allow vcspull
 
 ## vcspull v1.51.x (unreleased)
 
+<!-- KEEP THIS PLACEHOLDER - DO NOT REMOVE OR MODIFY THIS LINE -->
 _Notes on upcoming releases will be added here_
+<!-- END PLACEHOLDER - ADD NEW CHANGELOG ENTRIES BELOW THIS LINE -->
 
 <!-- Maintainers, insert changes / features for the next release here -->