feat: Add Security Inspector rules for IAM and Secrets Manager

[nathanhuh]

Summary

Add built-in Security Inspector rules for identity and secret hygiene checks.

Details

• Implement IAM access key age checks for stale keys
• Implement Secrets Manager rotation age checks
• Emit findings with clear remediation recommendations
• Add focused unit tests for each rule path

Checklist

• Add IAM access key age rules
• Add Secrets Manager rotation age rules
• Wire rules into the Inspector rule registry
• Add tests for passing and failing resource states

Related umbrella issue: #59
Depends on: #89

[nathanhuh]

@Unic-bot: assign me

[github-actions]

@nathanhuh has been assigned to this issue.