chore: add changelog and registry guidelines for module updates

.cursor/rules/changelog-and-registry.mdc

@@ -0,0 +1,7 @@
+---
+alwaysApply: true
+---
+
+when making changes to a module that has a changelog, add the appropriate unreleased change log entry
+
+when preparing an MPM plugin release, don't forget to update @public/registry.json