ponytail-help: Codex invocation should be $skill-name, not @skill-name

Thanks for ponytail. Reporting a docs mismatch (v4.10.0).

skills/ponytail-help/SKILL.md says "Codex uses @ponytail, @ponytail-review, and @ponytail-help". README.md line 340 says "invoke with @".

Per the Codex skills docs, explicit invocation is $skill-name, so @ won't invoke these skills. Suggested: $ponytail, $ponytail-review, $ponytail-help.

Happy to open a PR if you'd like.