Add OCI skill packager using ocispec types

Implement the SkillPackager interface with deterministic OCI artifact
creation from skill directories. Uses canonical ocispec types from
github.com/opencontainers/image-spec for OCI 1.1 compliance.

Reads SKILL.md YAML frontmatter for metadata, creates per-platform
OCI configs (ocispec.Image) with skill metadata in labels, manifests
(ocispec.Manifest) with annotations, and multi-platform image indexes
(ocispec.Index). Uses digest.FromBytes for diff IDs and
specs.Versioned for schema version.

Security: rejects symlinks (files and directories), hardlinks, device
entries, and path traversal in filesystem reads. Nil store panics at
construction. Frontmatter capped at 64KB.

New dependencies: github.com/opencontainers/image-spec, gopkg.in/yaml.v3

Resolves: #16

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

Author: JAORMX

go.mod

@@ -10,6 +10,7 @@ require (
 	github.com/stretchr/testify v1.11.1
 	go.uber.org/mock v0.6.0
 	golang.org/x/net v0.49.0
+	gopkg.in/yaml.v3 v3.0.1
 	oras.land/oras-go/v2 v2.6.0
 )
 
@@ -25,5 +26,4 @@ require (
 	google.golang.org/genproto/googleapis/api v0.0.0-20240826202546-f6391c0de4c7 // indirect
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20240826202546-f6391c0de4c7 // indirect
 	google.golang.org/protobuf v1.36.10 // indirect
-	gopkg.in/yaml.v3 v3.0.1 // indirect
 )