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/secrets_size_test.go

@@ -10,7 +10,6 @@ import (
 	"strings"
 	"testing"
 
-	"git.eeqj.de/sneak/secret/internal/secret"
 	"git.eeqj.de/sneak/secret/internal/vault"
 	"git.eeqj.de/sneak/secret/pkg/agehd"
 	"github.com/spf13/afero"
@@ -71,11 +70,8 @@ func newSizeTestVault(t *testing.T) (afero.Fs, *vault.Vault) {
 
 	fs := afero.NewMemMapFs()
 
-	// Set test mnemonic
-	t.Setenv(secret.EnvMnemonic, testMnemonic)
-
 	// Create vault
-	_, err := vault.CreateVault(fs, testStateDir, testVaultName)
+	_, err := vault.CreateVault(fs, testStateDir, testVaultName, testMnemonicBuffer(t))
 	require.NoError(t, err)
 
 	// Set current vault
@@ -205,7 +201,7 @@ func runImportSecretSizeCase(t *testing.T, size int, wantErr bool, errMsg string
 
 // TestAddSecretVariousSizes tests adding secrets of various sizes through stdin
 //
-//nolint:paralleltest // subtests use t.Setenv via newSizeTestVault
+//nolint:paralleltest // together the subtests lock more than the memlock limit
 func TestAddSecretVariousSizes(t *testing.T) {
 	tests := []struct {
 		name        string
@@ -265,7 +261,7 @@ func TestAddSecretVariousSizes(t *testing.T) {
 
 // TestImportSecretVariousSizes tests importing secrets of various sizes from files
 //
-//nolint:paralleltest // subtests use t.Setenv via newSizeTestVault
+//nolint:paralleltest // together the subtests lock more than the memlock limit
 func TestImportSecretVariousSizes(t *testing.T) {
 	tests := []struct {
 		name        string
@@ -325,7 +321,7 @@ func TestImportSecretVariousSizes(t *testing.T) {
 
 // TestAddSecretBufferGrowth tests that our buffer growth strategy works correctly
 //
-//nolint:paralleltest // subtests use t.Setenv via newSizeTestVault
+//nolint:paralleltest // together the subtests lock more than the memlock limit
 func TestAddSecretBufferGrowth(t *testing.T) {
 	// Test various sizes that should trigger buffer growth
 	sizes := []int{
@@ -392,9 +388,9 @@ func TestAddSecretBufferGrowth(t *testing.T) {
 }
 
 // TestAddSecretStreamingBehavior tests that we handle streaming input correctly
-//
-//nolint:paralleltest // uses t.Setenv via newSizeTestVault
 func TestAddSecretStreamingBehavior(t *testing.T) {
+	t.Parallel()
+
 	fs, vlt := newSizeTestVault(t)
 
 	// Create a custom reader that simulates slow streaming input