Starters should not constrain users to a specific package manager.
The .gitignore gets cloned into the user's project, so ignoring
certain lock files would prevent them from committing their chosen
package manager's lock file.
Flatten langgraph-python, langgraph-js, and mcp-apps starters so
npm install && npm run dev works out of the box. Replace Turborepo
with concurrently, move apps/* to root, update Dockerfiles, READMEs,
and entrypoints. Also remove stray pnpm-lock.yaml from a2a-a2ui and
ms-agent-framework-dotnet starters.