Print the mnemonic and the vault and unlocker listings to stdout (closes #138)

`secret generate mnemonic`, `secret vault list` and `secret unlocker list`,
in text and `--json` form, printed through cobra's `cmd.Println` and
`cmd.Printf`. Those write to stderr when no output writer is set, and the
binary sets none, so a pipe or `$(...)` got nothing. They now write to
`cmd.OutOrStdout()`, as `secret list` does, and so does "No versions found"
from `secret version list`. Status messages and the warning printed after
the mnemonic stay on stderr.

A new test runs the built binary and reads the stdout of `generate
mnemonic` and of both listings in both forms.

Model: opus-5-5

internal/cli/generate.go

@@ -103,7 +103,7 @@ func (cli *Instance) GenerateMnemonic(cmd *cobra.Command) error {
 	}
 
 	// Output mnemonic to stdout
-	cmd.Println(mnemonic)
+	_, _ = fmt.Fprintln(cmd.OutOrStdout(), mnemonic)
 
 	// Output helpful information to stderr
 	fmt.Fprintln(os.Stderr, "")