Name only the mnemonic when it cannot be read (closes #115)

secret init and secret vault create read the mnemonic with the new
secret.ReadMnemonic, whose every error wraps the new
secret.ErrMnemonicNotRead. It shares the terminal read with ReadPassphrase,
whose errors still wrap ErrPassphraseNotRead. Without a terminal the error
names the environment variable that gives the value instead:
SB_SECRET_MNEMONIC for the mnemonic, SB_UNLOCK_PASSPHRASE for the
passphrase. A test pins the message of init without a terminal.

Model: opus-5-5

internal/cli/init.go

@@ -55,11 +55,11 @@ func (cli *Instance) promptMnemonic() (*memguard.LockedBuffer, func(), error) {
 	secret.Debug("Prompting user for mnemonic phrase")
 
 	// Read mnemonic securely without echo
-	mnemonicBuffer, err := secret.ReadPassphrase("Enter your BIP39 mnemonic phrase: ")
+	mnemonicBuffer, err := secret.ReadMnemonic("Enter your BIP39 mnemonic phrase: ")
 	if err != nil {
 		secret.Debug("Failed to read mnemonic from stdin", "error", err)
 
-		return nil, nil, fmt.Errorf("failed to read mnemonic: %w", err)
+		return nil, nil, err
 	}
 
 	fmt.Fprintln(os.Stderr) // Add newline after hidden input