keyfork wizard generate-shard-secret: do not write to terminal

keyfork/src/cli/wizard.rs

@@ -1,6 +1,6 @@
 use super::Keyfork;
 use clap::{Parser, Subcommand};
-use std::collections::HashSet;
+use std::{collections::HashSet, io::IsTerminal};
 
 use card_backend_pcsc::PcscBackend;
 use openpgp_card_sequoia::{state::Open, types::KeyType, Card};
@@ -99,6 +99,11 @@ fn generate_shard_secret(threshold: u8, max: u8, keys_per_shard: u8) -> Result<(
     let mut pm = PromptManager::new(std::io::stdin(), std::io::stderr())?;
     let mut certs = vec![];
     let mut seen_cards: HashSet<String> = HashSet::new();
+    let stdout = std::io::stdout();
+    assert!(
+        !stdout.is_terminal(),
+        "not printing shard to terminal, redirect output"
+    );
 
     for index in 0..max {
         let cert = derive_key(&seed, index)?;