Improve access review guest and shared account gates

[danyili2632]

Summary

• add external guest evidence gates for sponsor status, business expiry, package expiry, nested group blind spots, and delegated app activity
• add shared-account attribution gates for PAM checkout, session recording, command/audit correlation, and post-use review
• extend output fields, platform evidence sources, severity escalation, and report categories for external/shared account findings

Bounty

Addresses #1521.

I have read and agree to the CONTRIBUTING.md bounty terms. Preferred payment method can be provided privately after maintainer acceptance.

Validation

• git diff --check