Add native sub-agent manifest endpoints

[Aaronontheweb]

Closes #89

Summary

• add native sub-agent manifest collection, identity, and version documents
• expose /manifest/subagents traversal routes from the root native manifest
• verify sub-agent manifest traversal and RFC skill feed isolation

Tests

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