fix: collections API should parse skills_json into structured response

[pavelanni]

From code review of PR #15.

GET /api/v1/collections/{name} returns skills_json as a JSON-encoded string inside JSON, which is awkward for API consumers:

{"skills_json": "[{\"name\":\"s1\",\"image\":\"quay.io/org/s1:1.0.0\"}]"}

Should return parsed structure:

{"skills": [{"name": "s1", "image": "quay.io/org/s1:1.0.0"}]}

Options:

• Add a Skills []SkillRef field to Collection with custom MarshalJSON
• Or use a response DTO that parses skills_json before serialization

Files: internal/store/store.go, internal/handler/collections.go