Add sub-agent storage and artifact endpoints

[Aaronontheweb]

Parent epic: #84

Add first-class SkillServer support for versioned NetClaw sub-agent definitions.

Tasks:

• Add sub-agent tables and migrations.
• Add sub-agent domain/API models.
• Add parser/validator for NetClaw sub-agent markdown.
• Add upload endpoint.
• Add metadata endpoints.
• Add agent.md artifact download endpoint.
• Add delete/version management behavior consistent with skills.

Acceptance criteria:

• Valid sub-agent markdown uploads successfully.
• Invalid frontmatter returns actionable errors.
• Duplicate versions return conflict.
• Artifact digest verifies downloaded agent.md bytes.