ci: replace clone-count with jgehrcke/github-repo-stats

ci: replace clone-count with jgehrcke/github-repo-stats

- Replace fragile MShawon/github-clone-count-badge (unversioned curl)
  with jgehrcke/github-repo-stats (SHA-pinned, proper deduplication)
- Add schneegans/dynamic-badges-action for shields.io endpoint badge
- Pin all actions to full commit SHA (supply chain hardening)
- Add permissions, concurrency, and timeout-minutes
- Remove obsolete CLONE.md
- Rename workflow file to repo-stats.yml