Remove all bundled polyglot-test-agent content (8 agents, 1 skill,
plugin metadata) and add dotnet-test as an external plugin from
dotnet/skills. The external plugin is a superset (11 agents, 22 skills).
Marketplace and doc indexes regenerated via npm run build.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>