Keep secret get values in locked memory (closes #37)

Vault.GetSecret and Vault.GetSecretVersion return the decrypted value
as a *memguard.LockedBuffer instead of copying it into an ordinary
[]byte that nothing wiped. Every caller destroys the buffer, and
`secret get` writes its bytes straight to stdout, still with no
trailing newline. Instance.Print, which formatted through fmt and had
no other callers, is removed, and so is a debug log line in
`get --version` that held the plaintext value.

Model: opus-5-5

internal/vault/secrets_version_test.go

@@ -131,7 +131,10 @@ func TestVaultAddSecretCreatesVersion(t *testing.T) {
 	// Get the secret value
 	retrievedValue, err := vault.GetSecret(testSecretPath)
 	require.NoError(t, err)
-	assert.Equal(t, expectedValue, retrievedValue)
+
+	defer retrievedValue.Destroy()
+
+	assert.Equal(t, expectedValue, retrievedValue.Bytes())
 }
 
 //nolint:paralleltest // createTestVaultWithKey uses t.Setenv
@@ -165,7 +168,10 @@ func TestVaultAddSecretMultipleVersions(t *testing.T) {
 	// Current value should be version-2
 	value, err := vault.GetSecret(testSecretPath)
 	require.NoError(t, err)
-	assert.Equal(t, []byte("version-2"), value)
+
+	defer value.Destroy()
+
+	assert.Equal(t, []byte("version-2"), value.Bytes())
 }
 
 //nolint:paralleltest // createTestVaultWithKey uses t.Setenv
@@ -192,15 +198,21 @@ func TestVaultGetSecretVersion(t *testing.T) {
 
 	// Get specific version (first one)
 	firstVersion := versions[1] // Last in list is first created
-	value, err := vault.GetSecretVersion(testSecretPath, firstVersion)
+	first, err := vault.GetSecretVersion(testSecretPath, firstVersion)
 	require.NoError(t, err)
-	assert.Equal(t, []byte("version-1"), value)
+
+	defer first.Destroy()
+
+	assert.Equal(t, []byte("version-1"), first.Bytes())
 
 	// Get specific version (second one)
 	secondVersion := versions[0] // First in list is most recent
-	value, err = vault.GetSecretVersion(testSecretPath, secondVersion)
+	second, err := vault.GetSecretVersion(testSecretPath, secondVersion)
 	require.NoError(t, err)
-	assert.Equal(t, []byte("version-2"), value)
+
+	defer second.Destroy()
+
+	assert.Equal(t, []byte("version-2"), second.Bytes())
 
 	// An empty version is not one of the versions; GetSecret gets the
 	// current one