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/secret/seunlocker_darwin.go

@@ -207,9 +207,12 @@ func generateSEKeyLabel(vaultName string) (string, error) {
 // CreateSecureEnclaveUnlocker creates a new SE unlocker.
 // The vault's long-term private key is encrypted directly by the Secure Enclave
 // using ECIES. No intermediate age keypair is used.
+// The long-term key comes from mnemonic when it is not nil, else from the
+// current unlocker, as getLongTermKeyForSE describes.
 func CreateSecureEnclaveUnlocker(
 	fs afero.Fs,
 	stateDir string,
+	mnemonic, passphrase *memguard.LockedBuffer,
 ) (*SecureEnclaveUnlocker, error) {
 	if err := checkMacOSAvailable(); err != nil {
 		return nil, err
@@ -236,7 +239,7 @@ func CreateSecureEnclaveUnlocker(
 	Debug("Created SE key", "label", seKeyLabel, "hash", seKeyHash)
 
 	// Step 2: Get the vault's long-term private key
-	ltPrivKeyData, err := getLongTermKeyForSE(fs, vault)
+	ltPrivKeyData, err := getLongTermKeyForSE(fs, vault, mnemonic, passphrase)
 	if err != nil {
 		return nil, fmt.Errorf(
 			"failed to get long-term private key: %w",
@@ -306,14 +309,15 @@ func CreateSecureEnclaveUnlocker(
 	}, nil
 }
 
-// getLongTermKeyForSE retrieves the vault's long-term private key
-// either from the mnemonic env var or by unlocking via the current unlocker.
+// getLongTermKeyForSE retrieves the vault's long-term private key, derived
+// from mnemonic when it is not nil, else through the current unlocker, which
+// is given passphrase when it is a passphrase unlocker.
 func getLongTermKeyForSE(
 	fs afero.Fs,
 	vault VaultInterface,
+	mnemonic, passphrase *memguard.LockedBuffer,
 ) (*memguard.LockedBuffer, error) {
-	envMnemonic := os.Getenv(EnvMnemonic)
-	if envMnemonic != "" {
+	if mnemonic != nil {
 		// Read vault metadata to get the correct derivation index
 		vaultDir, err := vault.GetDirectory()
 		if err != nil {
@@ -333,7 +337,7 @@ func getLongTermKeyForSE(
 
 		// Use mnemonic with the vault's actual derivation index
 		ltIdentity, err := agehd.DeriveIdentity(
-			envMnemonic,
+			mnemonic.String(),
 			metadata.DerivationIndex,
 		)
 
@@ -352,6 +356,10 @@ func getLongTermKeyForSE(
 		return nil, fmt.Errorf("failed to get current unlocker: %w", err)
 	}
 
+	if passphraseUnlocker, ok := currentUnlocker.(*PassphraseUnlocker); ok {
+		passphraseUnlocker.Passphrase = passphrase
+	}
+
 	currentIdentity, err := currentUnlocker.GetIdentity()
 	if err != nil {
 		return nil, fmt.Errorf(