feat(oci/skills): add DeleteTag and DeleteBuild for tag and artifact removal

[samuv]

Summary

• Adds DeleteTag(ctx, tag) which removes a tag entry from index.json via ORAS Untag without touching blobs; returns a 404-coded error when the tag does not exist
• Adds DeleteBuild(ctx, tag) which wraps DeleteTag with lightweight orphaned-blob cleanup: resolves the digest before untagging, then walks the content graph (index → manifests → config/layers) and removes blob files only when no remaining tag still references the same digest
• DeleteTag is kept as the low-level primitive for callers that explicitly want tag-only removal; DeleteBuild is the higher-level operation intended for the CLI (thv skill builds remove <tag>) where users expect the artifact to be fully gone

Test plan

• task passes (lint + tests green, race detector enabled)
• TestStore_DeleteTag: tag exists and is removed; tag does not exist returns 404; removing one tag does not affect sibling tags on the same digest
• TestStore_DeleteBuild: tag and blobs removed when digest is orphaned; blobs retained when another tag shares the digest; returns 404 when tag does not exist