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.
Nothing below the command reads the environment. README warns against
both variables.

Model: opus-5-5

internal/cli/vault.go

@@ -5,7 +5,6 @@ import (
 	"errors"
 	"fmt"
 	"log"
-	"os"
 	"path/filepath"
 	"slices"
 	"strings"
@@ -85,6 +84,9 @@ func newVaultCreateCmd() *cobra.Command {
 				return fmt.Errorf("failed to initialize CLI: %w", err)
 			}
 
+			destroySecrets := cli.readSecretEnv()
+			defer destroySecrets()
+
 			return cli.CreateVault(cmd, args[0])
 		},
 	}
@@ -136,6 +138,9 @@ func newVaultImportCmd() *cobra.Command {
 				return fmt.Errorf("failed to initialize CLI: %w", err)
 			}
 
+			destroySecrets := cli.readSecretEnv()
+			defer destroySecrets()
+
 			return cli.VaultImport(cmd, vaultName)
 		},
 	}
@@ -228,28 +233,14 @@ func (cli *Instance) ListVaults(cmd *cobra.Command, jsonOutput bool) error {
 	return nil
 }
 
-// setMnemonicEnv sets the mnemonic environment variable and returns a
-// function that restores the previous value
-func setMnemonicEnv(mnemonicStr string) func() {
-	originalMnemonic := os.Getenv(secret.EnvMnemonic)
-	_ = os.Setenv(secret.EnvMnemonic, mnemonicStr)
-
-	return func() {
-		if originalMnemonic != "" {
-			_ = os.Setenv(secret.EnvMnemonic, originalMnemonic)
-		} else {
-			_ = os.Unsetenv(secret.EnvMnemonic)
-		}
-	}
-}
-
-// resolvePassphrase returns the unlock passphrase from the environment or
-// prompts the user for it with confirmation
-func resolvePassphrase() (*memguard.LockedBuffer, error) {
-	if envPassphrase := os.Getenv(secret.EnvUnlockPassphrase); envPassphrase != "" {
+// resolvePassphrase returns the unlock passphrase from the environment,
+// cli.UnlockPassphrase, or prompts the user for it with confirmation. The
+// returned cleanup function must be deferred by the caller.
+func (cli *Instance) resolvePassphrase() (*memguard.LockedBuffer, func(), error) {
+	if cli.UnlockPassphrase != nil {
 		secret.Debug("Using unlock passphrase from environment variable")
 
-		return memguard.NewBufferFromBytes([]byte(envPassphrase)), nil
+		return cli.UnlockPassphrase, func() {}, nil
 	}
 
 	secret.Debug("Prompting user for unlock passphrase")
@@ -257,10 +248,10 @@ func resolvePassphrase() (*memguard.LockedBuffer, error) {
 	// Use secure passphrase input with confirmation
 	passphraseBuffer, err := readSecurePassphrase("Enter passphrase for unlocker: ")
 	if err != nil {
-		return nil, fmt.Errorf("failed to read passphrase: %w", err)
+		return nil, nil, fmt.Errorf("failed to read passphrase: %w", err)
 	}
 
-	return passphraseBuffer, nil
+	return passphraseBuffer, passphraseBuffer.Destroy, nil
 }
 
 // CreateVault creates a new vault
@@ -273,30 +264,13 @@ func (cli *Instance) CreateVault(cmd *cobra.Command, name string) error {
 	}
 	defer release()
 
-	// Get or prompt for mnemonic
-	var mnemonicStr string
-
-	if envMnemonic := os.Getenv(secret.EnvMnemonic); envMnemonic != "" {
-		secret.Debug("Using mnemonic from environment variable")
-
-		mnemonicStr = envMnemonic
-	} else {
-		secret.Debug("Prompting user for mnemonic phrase")
-		// Read mnemonic securely without echo
-		mnemonicBuffer, err := secret.ReadPassphrase(
-			"Enter your BIP39 mnemonic phrase: ")
-		if err != nil {
-			secret.Debug("Failed to read mnemonic from stdin", "error", err)
-
-			return fmt.Errorf("failed to read mnemonic: %w", err)
-		}
-		defer mnemonicBuffer.Destroy()
-
-		mnemonicStr = mnemonicBuffer.String()
-
-		fmt.Fprintln(os.Stderr) // Add newline after hidden input
+	mnemonic, cleanupMnemonic, err := cli.promptMnemonic()
+	if err != nil {
+		return err
 	}
+	defer cleanupMnemonic()
 
+	mnemonicStr := mnemonic.String()
 	if mnemonicStr == "" {
 		return errMnemonicEmpty
 	}
@@ -311,18 +285,14 @@ func (cli *Instance) CreateVault(cmd *cobra.Command, name string) error {
 
 	// Ask for the unlocker passphrase before creating the vault, so that
 	// stopping at the prompt leaves no vault without an unlocker behind
-	passphraseBuffer, err := resolvePassphrase()
+	passphraseBuffer, cleanupPassphrase, err := cli.resolvePassphrase()
 	if err != nil {
 		return err
 	}
-	defer passphraseBuffer.Destroy()
-
-	// Set mnemonic in environment for CreateVault to use
-	restoreMnemonicEnv := setMnemonicEnv(mnemonicStr)
-	defer restoreMnemonicEnv()
+	defer cleanupPassphrase()
 
 	// Create the vault - it will handle key derivation internally
-	vlt, err := vault.CreateVault(cli.fs, cli.stateDir, name)
+	vlt, err := vault.CreateVault(cli.fs, cli.stateDir, name, mnemonic)
 	if err != nil {
 		return err
 	}
@@ -412,11 +382,12 @@ func (cli *Instance) vaultImportPreflight(
 	}
 
 	// Get mnemonic from environment
-	mnemonic := os.Getenv(secret.EnvMnemonic)
-	if mnemonic == "" {
+	if cli.Mnemonic == nil {
 		return "", "", "", errMnemonicEnvNotSet
 	}
 
+	mnemonic := cli.Mnemonic.String()
+
 	// Validate the mnemonic
 	mnemonicWords := strings.Fields(mnemonic)
 	secret.Debug("Validating BIP39 mnemonic", "word_count", len(mnemonicWords))
@@ -534,17 +505,13 @@ func (cli *Instance) importMnemonic(cmd *cobra.Command, vaultName string) error
 	}
 
 	// Get passphrase from environment variable
-	passphraseStr := os.Getenv(secret.EnvUnlockPassphrase)
-	if passphraseStr == "" {
+	passphraseBuffer := cli.UnlockPassphrase
+	if passphraseBuffer == nil {
 		return errPassphraseEnvNotSet
 	}
 
 	secret.Debug("Using unlock passphrase from environment variable")
 
-	// Create secure buffer for passphrase
-	passphraseBuffer := memguard.NewBufferFromBytes([]byte(passphraseStr))
-	defer passphraseBuffer.Destroy()
-
 	// Unlock the vault with the derived long-term key
 	vlt.Unlock(ltIdentity)