Add privileged MFA assurance gates

[yanziwei]

Summary

Closes #1678.

• Adds privileged MFA assurance evidence gates to privileged-access.
• Adds PAM-MFA findings for weak MFA, missing step-up, push fatigue, recovery downgrade, vendor trust gaps, break-glass exceptions, and missing method-level audit evidence.
• Extends the output template with a privileged MFA assurance scorecard row and evidence table.
• Adds NIST SP 800-63B-4 and CISA phishing-resistant MFA references.
• Updates the skill version to 1.1.0.

Validation

• git diff --check
• Markdown fence balance check: 20 balanced
• Verified markers for version 1.1.0, Privileged MFA Assurance Evidence, PAM-MFA-01, phishing-resistant, and version history

Bounty request

Improver Moderate / $100 if accepted. Payment details can be provided privately after maintainer acceptance.