fix: add org.opencontainers.image.title annotation to skill layer descriptor

Without this annotation, `oras pull` skips the layer because it has no
filename to write to. Adding the title enables standard ORAS pull workflows.

Closes #70

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: JAORMX

oci/skills/packager.go

@@ -537,6 +537,9 @@ func createManifest(
 				MediaType: ocispec.MediaTypeImageLayerGzip,
 				Digest:    layerDigest,
 				Size:      int64(len(layerBytes)),
+				Annotations: map[string]string{
+					ocispec.AnnotationTitle: "skill.tar.gz",
+				},
 			},
 		},
 		Annotations: annotations,

oci/skills/packager_test.go

@@ -95,6 +95,7 @@ func TestPackager_Package_VerifyManifest(t *testing.T) {
 	assert.Equal(t, ocispec.MediaTypeImageConfig, manifest.Config.MediaType)
 	require.Len(t, manifest.Layers, 1)
 	assert.Equal(t, ocispec.MediaTypeImageLayerGzip, manifest.Layers[0].MediaType)
+	assert.Equal(t, "skill.tar.gz", manifest.Layers[0].Annotations[ocispec.AnnotationTitle])
 	assert.Equal(t, testSkillName, manifest.Annotations[AnnotationSkillName])
 }