Add OCI Image Spec reference and image manifest/image support to regshape-dev skill

[toddysm]

Summary

Update the regshape-dev skill so it can guide design and implementation of OCI image manifests and images (config + filesystem layers), and add an OCI Image Format Specification reference.

Changes

• Add a condensed references/oci-image-spec.md reference covering descriptors, image manifest, image index, image config (rootfs/diff_ids/history), filesystem layers (whiteouts, compression), media types, annotations, and an assemble/pull workflow.
• Mirror the full upstream OCI image-spec docs locally under references/oci-image-spec-source/ for authoritative reference.
• Update SKILL.md:
  • Phase 1 now points to both the distribution spec and the image spec.
  • Domain Areas add Image Manifests & Index (libs/manifests/) and Images (libs/images/).
  • Add image-spec and images issue labels.
  • Add the OCI Image Spec to Key References.

Acceptance Criteria

• references/oci-image-spec.md exists and is linked from SKILL.md.
• Upstream image-spec source is mirrored under references/oci-image-spec-source/.
• SKILL.md references the image spec in the workflow, domain areas, and key references.