Extend client native manifest support

[Aaronontheweb]

Closes #90

Summary

• add client native manifest models and link traversal helpers for skills and sub-agents
• add sub-agent upload, listing, download, and delete client methods
• add destination-agnostic verified artifact download helpers and adapter guidance

Tests

• dotnet build -c Release
• dotnet test -c Release
• pwsh ./scripts/Add-FileHeaders.ps1 -Verify
• dotnet slopwatch analyze
• git diff --check