docs(ai): Note moving changelog entries to Unreleased on rebase

A rebase onto main can land a branch after a release was cut, leaving a
new changelog entry under an already-released version heading. Document
that the entry should be moved back into an Unreleased section at the
top of CHANGELOG.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: runningcode

.cursor/rules/pr.mdc

@@ -93,6 +93,8 @@ Entry format:
 - <Short description> ([#<PR_NUMBER>](https://github.com/getsentry/sentry-java/pull/<PR_NUMBER>))
 ```
 
+**When rebasing:** A rebase onto `main` can land your branch after a release was cut, where the `## Unreleased` heading your entry lived under has since been renamed to that version number. If that happens, move your new entry into an `## Unreleased` section at the top of `CHANGELOG.md` (create the section if it no longer exists) so it is not left under an already-released version.
+
 Commit changelog separately:
 
 ```bash