docs: verify upstream sync completed before release

Add a checklist item to "Preparing the main Branch" pointing release
managers at dev/release/upstream-sync.md so the crate bump, dependency
consolidation, and /check-upstream and /audit-skill-md passes are
confirmed done before the release branch is cut.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer

dev/release/README.md

@@ -59,6 +59,9 @@ You will also need access to the [datafusion](https://test.pypi.org/project/data
 Before creating a new release:
 
 - We need to ensure that the main branch does not have any GitHub dependencies
+- Confirm the upstream sync workflow in [`upstream-sync.md`](upstream-sync.md) has been completed for this release cycle
+  (crate bump + breakage fixes, transitive dependency consolidation, and the `/check-upstream` and `/audit-skill-md`
+  passes). Any gaps surfaced by those skills should land before the release branch is cut.
 - a PR should be created and merged to update the major version number of the project
 - A new release branch should be created, such as `branch-53`
 - It is best to push this branch to the apache repository rather than a personal fork in case patch releases are required.