auth: say "login server" not "core" in user-facing text

Address PR review: auth use help said control-plane commands dial "the context's core" — reworded to "login server" (core is internal wording). Also reword the "build core API client" client-construction errors to "build Entire API client".

Scope New()'s doc comment correctly: the active context is used as-is for control-plane commands because they target a login server directly (no resource host to match against, unlike clone / the data API), and ENTIRE_AUTH_BASE_URL is the no-context fallback, not an override.