Add sub-agent collection to the native manifest

[Aaronontheweb]

Parent epic: #84

Expose sub-agents through the native manifest without adding them to the RFC skill feed.

Tasks:

• Add /manifest/subagents/index.json.
• Add path-based sub-agent pages.
• Add sub-agent identity index endpoints.
• Add sub-agent version detail endpoints.
• Link sub-agent collection from /manifest.json.
• Link skill version routesToSubagent when metadata.subagent is present.

Acceptance criteria:

• Native clients can traverse from root to sub-agent version detail.
• Sub-agent details use kind subagent-version and type agent-md.
• RFC index output remains skill-only.