packager: set OCI layout metadata for direct registry push

[sozercan]

Requires moby/buildkit#6574

Summary

• Set containerimage.oci-layout metadata key on BuildKit results from BuildModelpack() and BuildGeneric() (excluding files mode)
• This signals to a BuildKit image exporter with OCI layout passthrough support that the result contains a pre-built OCI layout and should be pushed as-is
• Preserves custom media types, annotations, artifactType, and full OCI artifact structure through the push

Dependencies

• Requires BuildKit change: feat: add OCI artifact export via postprocessor moby/buildkit#6574

How to test

# Create a buildx builder using the custom buildkit image
docker buildx create --name oci-test \
  --driver docker-container \
  --driver-opt image=sozercan/buildkit:oci-layout-passthrough

# Build and push a modelpack directly
docker buildx build \
  --builder oci-test \
  --build-arg BUILDKIT_SYNTAX=<aikit-frontend> \
  --target packager/modelpack \
  --build-arg source=huggingface://Qwen/Qwen3-0.6B \
  --build-arg name=qwen3 \
  --provenance=false --sbom=false \
  --output "type=image,name=docker.io/<user>/qwen3:0.6b,push=true" .

Verified

• Pushed docker.io/sozercan/qwen3:0.6b-test — manifest matches docker.io/sozercan/qwen3:0.6b exactly (flat manifest, same media types, same annotations)