[codex] fix skill-creator YAML frontmatter template

[chaochaoweb3]

Summary

• quote the generated name and TODO description values in init_skill.py
• reuse the existing YAML string quoting helper used for agents/openai.yaml
• add a small unittest that generated frontmatter parses description as a string

Why

The generated template used an unquoted bracketed TODO description. YAML parses that as a list, so a freshly initialized skill can fail validation before the user edits it. Quoting the template values keeps generated SKILL.md frontmatter valid and avoids colon/bracket parsing surprises.

Fixes #276.

Validation

• python3 -m py_compile init_skill.py test_init_skill.py quick_validate.py generate_openai_yaml.py
• python3 -m unittest test_init_skill.py
• generated a temporary skill with init_skill.py and validated it with quick_validate.py
• git diff --check