Epic: Transform to hephy-builder with multi-platform CI support

[kingdonb]

Epic Goal: Transform kaniko-builder into hephy-builder supporting multiple CI platforms and build backends

Background:
Evolution from single-purpose kaniko pipeline to comprehensive builder supporting the vision of "git push deis main again" - recreating the Deis Workflow experience with modern tooling.

Multi-Platform CI Support:

• GitHub Actions workflow equivalent to current GitLab CI
• Portable components that work in both platforms
• Configuration abstraction to support GitHub/GitLab/Enterprise/Self-hosted

Build Backend Matrix:

• Kaniko (current working solution)
• BuildKit (for advanced Dockerfile features)
• Ko (superior Go application builds)
• Spin (WebAssembly builds for SpinKube)

Configuration Evolution:

# Enhanced build-config.yaml
build_backend: kaniko|buildkit|ko|spin
ci_platform: gitlab|github
upstream_repo: https://github.com/owner/repo
# ... existing configuration

Success Criteria:

• Same build can run on GitHub Actions or GitLab CI
• Users can choose optimal build backend per project
• Maintains backward compatibility with existing configs
• Clear migration path from kaniko-builder to hephy-builder

Dependencies: Issue #3 (multi-target builds), current MVP functionality

Priority: High (defines next major version)