Read secret environment variables once per command, then unset them (closes #60)

init and vault create put the mnemonic into the process environment for
vault.CreateVault to read back, so every program they ran, gpg included,
inherited it, and SB_SECRET_MNEMONIC and SB_UNLOCK_PASSPHRASE were read
at 13 places and never unset. Each command that may need them now reads
both once, in its RunE, into locked buffers on the CLI Instance, and
unsets them at once. The buffers are passed down: vault.CreateVault
takes the mnemonic, a Vault carries Mnemonic and UnlockPassphrase, and
the PGP, keychain and Secure Enclave unlocker constructors take both;
CreatePGPUnlocker sets them on the vault it loads through SetMnemonic
and SetUnlockPassphrase, new in VaultInterface. README warns against
both variables.

Model: opus-5-5

internal/vault/vault_test.go

@@ -27,12 +27,19 @@ const (
 	testPassphrase = "test-passphrase"
 )
 
-//nolint:paralleltest // t.Setenv and order-dependent subtests forbid parallel
-func TestVaultOperations(t *testing.T) {
-	// Test environment will be cleaned up automatically by t.Setenv
-	t.Setenv(secret.EnvMnemonic, testMnemonic)
-	t.Setenv(secret.EnvUnlockPassphrase, testPassphrase)
+// testMnemonicBuffer returns testMnemonic in a locked buffer that is
+// destroyed when the test ends.
+func testMnemonicBuffer(t *testing.T) *memguard.LockedBuffer {
+	t.Helper()
 
+	mnemonic := memguard.NewBufferFromBytes([]byte(testMnemonic))
+	t.Cleanup(mnemonic.Destroy)
+
+	return mnemonic
+}
+
+//nolint:paralleltest // order-dependent subtests forbid parallel
+func TestVaultOperations(t *testing.T) {
 	// Use in-memory filesystem
 	fs := afero.NewMemMapFs()
 
@@ -64,7 +71,8 @@ func TestVaultOperations(t *testing.T) {
 func testCreateVault(t *testing.T, fs afero.Fs) {
 	t.Helper()
 
-	vlt, err := vault.CreateVault(fs, testStateDir, testVaultName)
+	vlt, err := vault.CreateVault(fs, testStateDir, testVaultName,
+		testMnemonicBuffer(t))
 	if err != nil {
 		t.Fatalf("Failed to create vault: %v", err)
 	}
@@ -221,6 +229,8 @@ func testUnlockerOperations(t *testing.T, fs afero.Fs) {
 	}
 
 	// Test vault unlocking (should happen automatically via mnemonic)
+	vlt.Mnemonic = testMnemonicBuffer(t)
+
 	if vlt.Locked() {
 		_, err := vlt.UnlockVault()
 		if err != nil {
@@ -281,15 +291,14 @@ func testUnlockerOperations(t *testing.T, fs afero.Fs) {
 }
 
 func TestListUnlockers_SkipsMissingMetadata(t *testing.T) {
-	// Set test environment variables
-	t.Setenv(secret.EnvMnemonic, testMnemonic)
-	t.Setenv(secret.EnvUnlockPassphrase, testPassphrase)
+	t.Parallel()
 
 	// Use in-memory filesystem
 	fs := afero.NewMemMapFs()
 
 	// Create vault
-	vlt, err := vault.CreateVault(fs, testStateDir, testVaultName)
+	vlt, err := vault.CreateVault(fs, testStateDir, testVaultName,
+		testMnemonicBuffer(t))
 	if err != nil {
 		t.Fatalf("Failed to create vault: %v", err)
 	}