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/unlock_failure_test.go

@@ -260,9 +260,9 @@ func TestPassphraseNotReadNamesNoMnemonic(t *testing.T) {
 
 	assert.Equal(t, "Error: failed to unlock vault: "+
 		"failed to get long-term key: failed to get unlocker identity: "+
-		"failed to read passphrase: cannot read passphrase from non-terminal "+
-		"stdin (piped input or script). Please set the SB_UNLOCK_PASSPHRASE "+
-		"environment variable or run interactively\n", string(output))
+		"failed to read passphrase: stdin is not a terminal (piped input or "+
+		"script). Please set the SB_UNLOCK_PASSPHRASE environment variable or "+
+		"run interactively\n", string(output))
 }
 
 // TestCryptoUnlockFailureNamesMnemonic checks that `secret encrypt` and