Add admin impersonation review skill

[xiefuzheng713-alt]

/claim #298

Summary

• Add a new admin-impersonation-feature-review identity/auth skill for SaaS support and admin login-as-user flows.
• Cover approval, reason capture, visible session marking, action restrictions, actor/target audit trails, tenant boundaries, revocation, and timeout behavior.
• Include README, pattern references, three vulnerable fixtures, three benign fixtures, and index registration.

Validation

• git diff --check
• git diff --cached --check
• required frontmatter check across skills/ and roles/
• index.yaml listed-file existence check
• skill_count consistency check
• prompt-injection pattern scan equivalent to the repository workflow
• python -m py_compile for the new Python fixtures, with generated cache files removed before commit
• node --check for the new JavaScript fixtures

Safety

• Defensive review guidance only.
• Synthetic fixtures only.
• No credentials, private keys, tokens, cookies, real customer data, payment credentials, or external API calls.

Closes #298