Add pluggable registry adapter pattern for enumerating and scanning all
container images in a private registry. Supports:
- Generic OCI registries (Harbor, GitLab, JFrog) via /v2/_catalog
- Docker Hub via Hub REST API with namespace-based enumeration
- AWS ECR via standard OCI endpoints with ECR auth token
New CLI flags: --registry, --image-filter, --tag-filter, --max-images,
--registry-insecure
