feat: add JSON Schema for SkillCollection YAML

Summary

SkillCard has a JSON Schema at schemas/skillcard.schema.json
that powers skillctl validate and enables IDE autocompletion.
SkillCollection YAML has no equivalent schema — there's no
validation beyond Go struct unmarshaling.

Proposed solution

Add schemas/skillcollection.schema.json covering:

• apiVersion (required, must be skillimage.io/v1alpha1)
• kind (required, must be SkillCollection)
• metadata.name, metadata.version, metadata.description
• skills[] entries with ref (and source once #35 lands)

Extend skillctl validate to detect collection files
(by kind: SkillCollection) and validate against the
collection schema.

Motivation

• Consistency with SkillCard validation
• Catch typos and missing fields early
• IDE autocompletion for collection authors
• Required foundation for #35 (Git source URLs in collections)

References

• Existing schema: schemas/skillcard.schema.json
• Collection implementation: pkg/collection/

Compare schemas/skillcard.schema.json with the collection implementation in pkg/collection/ and inspect the existing skillctl validate entry point. Define schemas/skillcollection.schema.json for the listed fields, including skills entries, then extend validation to recognize kind: SkillCollection and validate it; account for source once #35 lands.