test(skills): declare PyYAML so the standalone suite runs as documented (#8067)

CONTRIBUTING documents the public skill tests as a self-contained suite:

    python -m pip install -r tests/skills/requirements.txt
    python -m pytest tests/skills -q

That install is short one dependency. skills/public/skill-creator/scripts
imports PyYAML at module level (quick_validate.py, utils.py), and its SKILL.md
already states PyYAML is required, but tests/skills/requirements.txt never
listed it. A fresh environment built from that file therefore fails 34 tests
with ModuleNotFoundError: No module named 'yaml' instead of running the suite -
test_skill_packager_cli.py, test_skill_creator_frontmatter.py and
test_skill_creator_eval_output.py all drive the skill's CLI in a subprocess,
so every one of their cases reports a traceback.

CI does not see this because skill-review-ci.yml runs the suite through
`uv run --locked` inside the backend environment, which already resolves
PyYAML transitively. Only the documented local path is affected.

Pin the version the backend lockfile already resolves (uv.lock: >=6.0.3).

tests/skills/requirements.txt

@@ -4,4 +4,5 @@ openpyxl==3.1.5
 Pillow==13.0.0
 pytest==9.1.1
 python-pptx==1.0.2
+PyYAML==6.0.3
 requests==2.34.2