Create guide and evals for the privacy use case

[chrome-devrel-infra]

Action-oriented guidelines for web developers to implement privacy by design, data minimization, third-party audits, and secure data handling. Use this skill when designing applications, integrating third-party services, handling user data, or configuring security headers.

Affected web-feature IDs: permissions-policy, partitioned-cookies, fedcm, ua-client-hints

Use case subdir: guides/privacy/privacy

Related features: #1068

Required files:

• Use case metadata (guide.md frontmatter)
• demo.html
• Use case guidance (guide.md)
• expectations.md
• tasks/task.md
• negative-demo.html
• grader.ts