fix internal repo sync broken by actions/checkout v6 bump

#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>

.github/workflows/sync-internal.yaml

@@ -17,6 +17,9 @@ jobs:
         uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
         with:
           fetch-depth: 0
+          # Don't persist the checkout auth header; the push below authenticates
+          # with the app token embedded in the remote URL instead
+          persist-credentials: false
       - name: Generate GitHub App Token
         id: app-token
         uses: actions/create-github-app-token@bcd2ba49218906704ab6c1aa796996da409d3eb1 # v3.2.0
@@ -33,6 +36,5 @@ jobs:
         run: |
           git config user.name "github-actions"
           git config user.email "test@users.noreply.github.com"
-          git config --unset-all http.https://github.com/.extraheader
           git remote add internal https://x-access-token:${TOKEN}@github.com/bluesky-social/social-app-internal.git
           git push internal main --force