Implement /manifest.json native manifest root and skill collection

[Aaronontheweb]

Parent epic: #84

Implement the initial native manifest tree for skills.

Tasks:

• Add /manifest.json root endpoint.
• Add /manifest/skills/index.json.
• Add path-based skills pages.
• Add skill identity index endpoints.
• Add skill version detail endpoints.
• Add source-generated JSON models.

Acceptance criteria:

• Clients can traverse from /manifest.json to a skill version detail by following href values.
• Skill version detail reuses the RFC artifact fields and values.
• Unsupported future collections can be ignored by clients.

[Aaronontheweb]

Completed by #96.