docs/security/new-members-onboarding.md: stale .claude/skills/ reference + wrong link target #870

What should happen

1. Replace the .claude/skills/ display path with the canonical .agents/skills/ path.
2. Fix the cross-reference that sends new members to the README for role info — it should point to docs/security/roles.md.

Why

The framework's canonical skill path is .agents/skills/ (with relay symlinks from .claude/skills/). The doc uses the old path. Separately, new members looking for role descriptions are directed to the README, but that info lives in roles.md.

Which layer

Documentation (docs/, README.md, CONTRIBUTING.md)