Unknown subcommands of vault, unlocker, version and generate fail (closes #183)

Given a subcommand they do not have, `secret vault`, `secret unlocker`,
`secret version` and `secret generate` printed their help and exited 0,
so a mistyped command looked like a successful one. Each now exits 1
with `unknown command "<name>" for "secret <parent>"`, and with no
arguments still prints its help.

Cobra checks a command's Args only when it has a RunE, so each parent
gets `Args: cobra.NoArgs` and a RunE that prints its help. Args alone
changes nothing. TestNoVaultImportCommand required the old answer to
`secret vault import` and now requires the error.

Judgement call: their help now shows a `secret <parent> [flags]` usage
line, which cobra adds for any command that runs.
Judgement call: `secret vault help` now fails as an unknown subcommand;
`secret help vault` and `secret vault --help` still work.

Model: opus-5-5

internal/cli/create_vault_test.go

@@ -144,25 +144,20 @@ func TestVaultCreationLeavesNoSecretInEnvironment(t *testing.T) {
 // TestNoVaultImportCommand is a regression test for
 // https://git.eeqj.de/sneak/secret/issues/139: `secret vault import`
 // refused every vault that `secret vault create` makes, because each has a
-// long-term key. The command is gone, so cobra answers it with the help of
-// `secret vault`, as it does any subcommand of `secret vault` it does not
-// know.
+// long-term key. The command is gone, so it fails as any subcommand of
+// `secret vault` that does not exist does.
 func TestNoVaultImportCommand(t *testing.T) {
 	t.Setenv(secret.EnvStateDir, t.TempDir())
+	t.Setenv(secret.EnvMnemonic, testMnemonic)
+	t.Setenv(secret.EnvUnlockPassphrase, testPassphrase)
 
-	run := func(args ...string) string {
-		t.Setenv(secret.EnvMnemonic, testMnemonic)
-		t.Setenv(secret.EnvUnlockPassphrase, testPassphrase)
+	_, err := cli.ExecuteCommandInProcess(
+		strings.Fields("vault create work"), "", nil)
+	require.NoError(t, err)
 
-		output, err := cli.ExecuteCommandInProcess(args, "", nil)
-		require.NoError(t, err, "%v", args)
-
-		return output
-	}
-
-	run("vault", "create", "work")
-
-	require.Equal(t, run("vault"), run("vault", "import", "work"))
+	_, err = cli.ExecuteCommandInProcess(
+		strings.Fields("vault import work"), "", nil)
+	require.EqualError(t, err, `unknown command "import" for "secret vault"`)
 }
 
 // TestStopAtPassphrasePromptLeavesNothing is a regression test for the