Remove cbk and mnemonic commands from sutil

sutil holds only the utilities wanted on every machine; it is not a
command-line front end for the packages in pkg/. cbk and bip39wordlist
remain libraries with no CLI surface. The README states the boundary.

internal/cli/cli.go

@@ -37,9 +37,8 @@ func New(p Params) *cobra.Command {
 	root := &cobra.Command{
 		Use:   "sutil",
 		Short: "sneak's shell utility",
-		Long: `sutil is a small collection of shell utilities built on the packages in
-sneak.berlin/go/x: fast pseudorandom byte streams, memorable passwords,
-BIP39 mnemonics, and crypto_box keypairs derived from them.
+		Long: `sutil is a small collection of shell utilities worth having on every
+machine: a fast pseudorandom byte stream and a memorable password generator.
 
 Before any command runs, sutil loads the optional configuration file named
 by --config, or else ~/.sneak.yaml (or .json, .toml, ...) if it exists, and
@@ -58,7 +57,6 @@ makes environment variables available alongside it.`,
 		newVersionCommand(p),
 		newInfoCommand(a),
 		newGenerateCommand(),
-		newCBKCommand(),
 	)
 
 	return root