Fix skill-creator scaffold frontmatter

[wsk-builds]

Problem

skills/.system/skill-creator/scripts/init_skill.py generates a SKILL.md whose frontmatter description placeholder is written as a YAML list:

description: [TODO: ...]

That means a freshly generated skill fails validation immediately instead of producing a valid starter template.

Root Cause

The scaffold uses square-bracket YAML syntax for the placeholder, so description is parsed as a list rather than a string. The validator then rejects the generated file with:

Description must be a string, got list

Fix

• quote the generated description placeholder so it is parsed as a YAML string
• add a regression test that generates a new skill and validates it immediately

Validation

• python3 skills/.system/skill-creator/tests/test_init_skill.py
• python3 skills/.system/skill-creator/scripts/init_skill.py repro-skill-final --path /tmp
• python3 skills/.system/skill-creator/scripts/quick_validate.py /tmp/repro-skill-final

[wsk-builds]

Minimal repro for the current bug:

python3 skills/.system/skill-creator/scripts/init_skill.py repro-skill --path /tmp
python3 skills/.system/skill-creator/scripts/quick_validate.py /tmp/repro-skill

Before this change, the second command fails with:

Description must be a string, got list

The generated SKILL.md contains this frontmatter entry:

description: [TODO: ...]

which YAML parses as a list instead of a string.