Extend Netclaw.SkillClient for native manifest and sub-agents

[Aaronontheweb]

Parent epic: #84

Add typed client support for native manifest traversal and sub-agent artifact operations.

Tasks:

• Add native manifest models.
• Add GetManifestAsync.
• Add skill manifest traversal helpers.
• Add sub-agent manifest traversal helpers.
• Add upload/download methods for sub-agent definitions.
• Add destination-agnostic helpers for verified artifact download.
• Document how non-NetClaw clients can provide destination and format adapters.
• Ensure helper APIs do not assume NetClaw application paths or formats.
• Preserve AOT source-generated serialization.

Acceptance criteria:

• Client can fetch root manifest and follow collection links.
• Client can download a sub-agent artifact and verify digest.
• Client APIs do not hard-code NetClaw application paths.
• Client APIs can be reused by an OpenCode or other non-NetClaw sync adapter.
• Existing RFC client methods remain unchanged.