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/generate.go

@@ -29,6 +29,12 @@ func newGenerateCmd() *cobra.Command {
 		Use:   "generate",
 		Short: "Generate random data",
 		Long:  `Generate various types of random data including mnemonics and secrets.`,
+		// Cobra checks Args only on a command that has a RunE; without
+		// one, an unknown subcommand prints this help and exits 0.
+		Args: cobra.NoArgs,
+		RunE: func(cmd *cobra.Command, _ []string) error {
+			return cmd.Help()
+		},
 	}
 
 	cmd.AddCommand(newGenerateMnemonicCmd())