Define native manifest and sub-agent sync specifications

[Aaronontheweb]

Parent epic: #84

Create the public design specs for SkillServer native sync.

Tasks:

• Define AgentSkills.io-compatible skill package rules.
• Define NetClaw sub-agent package rules.
• Define /manifest.json native feed structure.
• Document metadata.subagent as a route, not a dependency.
• Link specs from README.

Acceptance criteria:

• Specs are committed under docs/specs/.
• README links to the specs.
• Current implementation gaps are called out explicitly.