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/cli/create_vault_test.go

@@ -1,6 +1,7 @@
 package cli_test
 
 import (
+	"bytes"
 	"testing"
 
 	"git.eeqj.de/sneak/secret/internal/cli"
@@ -89,7 +90,11 @@ func TestCreateExistingVaultChangesNothing(t *testing.T) {
 	for _, name := range vaults {
 		value, err := vault.NewVault(fs, testStateDir, name).GetSecret("x")
 		require.NoError(t, err)
-		require.Equal(t, "value", string(value))
+
+		unchanged := bytes.Equal([]byte("value"), value.Bytes())
+		value.Destroy()
+
+		require.True(t, unchanged, "vault %q kept its secret", name)
 	}
 }