As cyber threats evolve toward supply chain attacks, the “Shift Left” philoso-phy must transition from a recommendation to an enforced mechanical constraint. This paper presents a framework for a Secure-by-Default CI/CD pipeline utiliz-ing custom Golang-based admission controllers and Build-Breaker logic. I detail the automated integration of image hardening via distroless migrations and the mandatory enforcement of strict security headers—specifically Content Security Policy (CSP), X-Frame-Options (XFO), and HTTP Strict Transport Se-curity (HSTS). Through a high-fidelity simulation environment, I demonstrate that mechanical enforcement via build-breakers achieves 100% policy compliance while introducing manageable latency to the developer workflow.