fix: move reusable workflows to .github/workflows/

GitHub requires reusable workflows to be in .github/workflows/ directory.
Moved security and kubernetes workflows from workflows/ to correct location.

Moved files:
- workflows/security/gitleaks-scan.yml → .github/workflows/security/
- workflows/security/trivy-scan.yml → .github/workflows/security/
- workflows/security/sbom-generate.yml → .github/workflows/security/
- workflows/kubernetes/helm-publish.yml → .github/workflows/kubernetes/

This fixes CI/CD pipelines in ai-dev-env that reference these workflows.

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: samuelho-dev