Issue Description

The repository needs reusable workflow files for security tasks including dependency pinning validation, SHA staleness checking, secret scanning, and IaC security scanning.

Additional Context

Files to add:

• .github/workflows/reusable-dependency-pinning.yml - Validate dependencies are pinned to SHAs
• .github/workflows/reusable-sha-staleness.yml - Check for stale SHA references
• .github/workflows/reusable-gitleaks.yml - Secret scanning with Gitleaks
• .github/workflows/reusable-checkov.yml - IaC security scanning with Checkov
• .github/workflows/reusable-codeql.yml - CodeQL security analysis

Priority: Wave 3 (Priority 3) - Depends on Wave 2 completion

Dependencies: Requires security scripts from Issue #9, configurations from Issue #3

Testing: Validate YAML syntax, test each reusable workflow with sample inputs, verify security scans detect known issues