Fix missing-key errors for restore and deep verify to name both key sources (closes #401)

The missing-key error for restore and deep verify is one value, errDecryptionKeyRequired, whose text names both places the key can be set: the VAULTIK_AGE_SECRET_KEY environment variable first, then the age_secret_key config file field.

Model: deepseek-v4-pro

internal/vaultik/restore_identity_test.go

@@ -106,3 +106,23 @@ func TestRestoreIdentitiesMissingKey(t *testing.T) {
 	_, err := v.restoreIdentities()
 	require.ErrorIs(t, err, errDecryptionKeyRequired)
 }
+
+// TestMissingKeyErrorsNameBothSources asserts restore and deep verify
+// return the same missing-key error, whose text names both places the key
+// can be set: the VAULTIK_AGE_SECRET_KEY environment variable first since
+// it is preferred, then the age_secret_key config file field.
+func TestMissingKeyErrorsNameBothSources(t *testing.T) {
+	t.Parallel()
+
+	const want = "age secret key is missing; set it in " +
+		"the VAULTIK_AGE_SECRET_KEY environment variable or the " +
+		"age_secret_key config file field"
+
+	v := &Vaultik{Config: &config.Config{}}
+
+	_, err := v.restoreIdentities()
+	require.EqualError(t, err, want)
+
+	err = v.RunDeepVerify("any-snapshot", &VerifyOptions{})
+	require.EqualError(t, err, want)
+}