Extends every push-to-main CI workflow to also trigger on release-*
branches, so a backport merged to a release branch gets the same
post-merge test coverage main gets. Pull request triggers are already
branch-unfiltered, so backport PRs were covered; the gap was the branch
state after merging.
sync-main is deliberately untouched: it syncs main into a downstream
repository and must never run for a release branch.