ci(repo-stats): clone the github-repo-stats data branch, not gh-pages

    The opaque "exit code 1" from actions/checkout was actually masking a
    "Remote branch gh-pages not found in upstream origin" — the bambuddy
    repo doesn't have a gh-pages branch. jgehrcke/github-repo-stats writes
    to a branch called `github-repo-stats` by default, and Pages is wired
    to serve from that branch. The path under it (maziggy/bambuddy/latest-
    report/report.html) is unchanged.

    Switching the clone branch and renaming the local path from `gh-pages/`
    to `data/` so the workflow reads correctly. The PAT-in-URL pattern and
    the direct git clone (vs actions/checkout) stay — both still wanted so
    the real git stderr reaches the log if anything goes wrong.

    Verified by cloning `github-repo-stats` locally and running the
    injector against its live report.html: clean patch, 30 days extracted,
    all anchor markers (TOC, section, script) present.