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/cli/crypto.go

@@ -41,6 +41,9 @@ func newCryptoCmd(
 
 			cli.cmd = cmd
 
+			destroySecrets := cli.readSecretEnv()
+			defer destroySecrets()
+
 			return run(cli, args[0], inputFile, outputFile)
 		},
 	}
@@ -156,6 +159,8 @@ func (cli *Instance) Encrypt(secretName, inputFile, outputFile string) error {
 		return err
 	}
 
+	vlt.Mnemonic, vlt.UnlockPassphrase = cli.Mnemonic, cli.UnlockPassphrase
+
 	// Get or create the age secret key for this secret
 	keyBuffer, err := cli.resolveEncryptionKey(vlt, secretName)
 	if err != nil {
@@ -230,6 +235,8 @@ func (cli *Instance) Decrypt(secretName, inputFile, outputFile string) error {
 		return err
 	}
 
+	vlt.Mnemonic, vlt.UnlockPassphrase = cli.Mnemonic, cli.UnlockPassphrase
+
 	// Check if secret exists
 	secretObj := secret.NewSecret(vlt, secretName)
 
@@ -308,13 +315,13 @@ func isValidAgeSecretKey(key string) bool {
 	return err == nil
 }
 
-// getSecretValue retrieves the value of a secret using the appropriate
-// unlocker
+// getSecretValue retrieves the value of a secret with the vault's mnemonic
+// when it has one, else with the current unlocker
 func (cli *Instance) getSecretValue(
 	vlt *vault.Vault, secretObj *secret.Secret,
 ) (*memguard.LockedBuffer, error) {
-	if os.Getenv(secret.EnvMnemonic) != "" {
-		return secretObj.GetValue(nil)
+	if vlt.Mnemonic != nil {
+		return secretObj.GetValue(nil, vlt.Mnemonic)
 	}
 
 	unlocker, err := vlt.GetCurrentUnlocker()
@@ -322,5 +329,5 @@ func (cli *Instance) getSecretValue(
 		return nil, fmt.Errorf("failed to get current unlocker: %w", err)
 	}
 
-	return secretObj.GetValue(unlocker)
+	return secretObj.GetValue(unlocker, nil)
 }