feat(actions): add reusable gitleaks PR scan workflow

[anmolnagpal]

Summary

• add reusable workflow .github/workflows/gitleaks-pr-scan.yml
• expose workflow_call interface for org repos
• run checkout + gitleaks scan with GitHub token

Why

• centralize gitleaks implementation in shared workflows
• allow terraform/module repos to only call shared workflow without embedding full logic

Notes

• intended for downstream usage from org .github and terraform module repos

[anmolnagpal]

Ready to merge from my side ✅\n\nChecks are green and approvals are in place. I attempted merge but branch protection permissions for my token do not allow merge/auto-merge on this protected branch.\n\nOnce merged, I will immediately do tag release with changelog as requested by Anmol.