skill-creator: quick_validate.py imports PyYAML but it's not declared as a dependency anywhere

scripts/quick_validate.py does:

import yaml
...
frontmatter = yaml.safe_load(frontmatter_text)
...
except yaml.YAMLError as e:

yaml (PyYAML) is not part of the Python standard library, and neither skill-creator/ nor the repo root declares it as a dependency anywhere (no requirements.txt, pyproject.toml, etc. in this repo). On a machine that doesn't happen to have PyYAML already installed globally, running the skill's validation step fails with ModuleNotFoundError: No module named 'yaml'.

We hit this while vendoring skill-creator into an internal docs repo and auditing which external packages our agent skills actually require — this was the only "silently required but undeclared" dependency we found.

Suggested fix: add a requirements.txt (or equivalent) alongside skill-creator/scripts/quick_validate.py pinning pyyaml, so anyone installing/running this skill standalone (e.g. via gh skill install) knows to install it first.

Related but distinct from #150 (which is about whether skill frontmatter itself should support a dependencies field) — this issue is specifically about the validator script's own missing dependency declaration.