ci(repo-stats): use jgehrcke's PAT for gh-pages checkout

    The default GITHUB_TOKEN failed at `git fetch` for the gh-pages
    checkout step with opaque "exit code 1" and no surfaced git stderr,
    even with permissions: contents: write set at workflow level.
    actions/checkout's retry loop didn't recover.

    Switching the gh-pages checkout to secrets.GHRS_GITHUB_API_TOKEN —
    the same PAT jgehrcke/github-repo-stats already writes the branch
    with one step earlier — keeps the auth chain uniform and avoids the
    mismatch. persist-credentials defaults to true, so the subsequent
    commit-and-push step in the same gh-pages directory picks up the
    PAT automatically; no separate change to the push step needed.

    fetch-depth: 1 left explicit because checkout@v4 defaults to it but
    the value's load-bearing for this workflow (we only need HEAD of
    gh-pages, not history).