[Enhancement] Add Explicit Validation Loop and Actionable Checklist to Security Review Phase

[santosomar]

Enhance the Security Review phase in Project CodeGuard by:

Adding an explicit validation/feedback loop (e.g., “If issues found: fix and re-review before committing”).

Introducing a concise, actionable checklist format to make verification clearer and more consistent.

These improvements are aligned with feedback from the Tessl evaluation of CodeGuard skills:
https://tessl.io/registry/cisco/software-security/skills