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/metadata_test.go

@@ -297,14 +297,14 @@ func TestSampleHashCalculation(t *testing.T) {
 }
 
 func TestWorkflowMismatch(t *testing.T) {
+	t.Parallel()
+
 	// Create a temporary directory for testing
 	tempDir := t.TempDir()
 	fs := afero.NewOsFs()
 
 	// Test Case 1: Create vault WITH mnemonic (like init command)
-	t.Setenv("SB_SECRET_MNEMONIC", testMnemonic)
-
-	_, err := vault.CreateVault(fs, tempDir, "default")
+	_, err := vault.CreateVault(fs, tempDir, "default", testMnemonicBuffer(t))
 	if err != nil {
 		t.Fatalf("Failed to create vault with mnemonic: %v", err)
 	}
@@ -321,19 +321,15 @@ func TestWorkflowMismatch(t *testing.T) {
 		metadata1.DerivationIndex, metadata1.PublicKeyHash)
 
 	// Test Case 2: Create vault WITHOUT mnemonic, then import (work vault)
-	t.Setenv("SB_SECRET_MNEMONIC", "")
-
-	_, err = vault.CreateVault(fs, tempDir, "work")
+	_, err = vault.CreateVault(fs, tempDir, "work", nil)
 	if err != nil {
 		t.Fatalf("Failed to create vault without mnemonic: %v", err)
 	}
 
 	vault2Dir := filepath.Join(tempDir, "vaults.d", "work")
 
-	// Simulate the vault import process
-	t.Setenv("SB_SECRET_MNEMONIC", testMnemonic)
-
-	// Get the next available derivation index for this mnemonic
+	// Simulate the vault import process: get the next available derivation
+	// index for this mnemonic
 	derivationIndex, err := vault.GetNextDerivationIndex(fs, tempDir, testMnemonic)
 	if err != nil {
 		t.Fatalf("Failed to get next derivation index: %v", err)