Fix Windows validation dependencies

[icidade]

What changed

• installs PyYAML==6.0.3 before repository validation on Linux and Windows
• makes YAML validation deterministic across both CI jobs

Root cause

The PowerShell validator imports yaml unconditionally, but the Windows runner did not have PyYAML installed. This produced ModuleNotFoundError: No module named 'yaml' and incorrectly reported config/chief-of-staff.public.yaml as invalid.

Validation

• git diff --check passed
• the workflow dependency version was verified against the current stable PyYAML release on PyPI
• GitHub Actions will provide the cross-platform verification for this workflow-only change