#10779 bumped actions/checkout v5 -> v6. v6 no longer stores the auth
header as a local http.extraheader git config entry, so the
`git config --unset-all http.https://github.com/.extraheader` call in the
push step found nothing to unset and exited with code 5, failing every
sync since.
Set persist-credentials: false on checkout so no auth header is injected,
and drop the now-unnecessary (and failing) unset. The push to the internal
repo authenticates via the app token embedded in the remote URL.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Samuel Newman