Provider model

The root CLI is provider-neutral, while every integration owns its commands, services, and types. Today the root registers one utility command and one provider namespace:

vac
├── upgrade
└── retell

The boundary is concrete:

Path	Responsibility
src/cli.ts	Defines vac and registers root commands and providers
src/commands/	Provider-neutral user commands such as self-upgrade
src/core/	Provider-neutral parsing, pagination, and error helpers
src/providers/retell/	Retell commands, configuration, SDK access, prompts, and types

src/architecture.test.ts fails if a retell-sdk import appears outside src/providers/retell/.

There is no runtime plugin system, provider interface, or second provider. A future provider should add its own src/providers/<name>/ module and explicit root registration. Shared abstractions should wait until two real providers expose the same behavior.