test: add test for getLongTermPrivateKey derivation index

Verifies that getLongTermPrivateKey reads the derivation index from
vault metadata instead of using hardcoded index 0. Test creates a
mock vault with DerivationIndex=5 and confirms the derived key
matches index 5.

internal/secret/derivation_index_test.go

@@ -0,0 +1,82 @@
+package secret
+
+import (
+	"encoding/json"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"git.eeqj.de/sneak/secret/pkg/agehd"
+	"github.com/awnumar/memguard"
+	"github.com/spf13/afero"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// realVault is a minimal VaultInterface backed by a real afero filesystem,
+// using the same directory layout as vault.Vault.
+type realVault struct {
+	name     string
+	stateDir string
+	fs       afero.Fs
+}
+
+func (v *realVault) GetDirectory() (string, error) {
+	return filepath.Join(v.stateDir, "vaults.d", v.name), nil
+}
+func (v *realVault) GetName() string        { return v.name }
+func (v *realVault) GetFilesystem() afero.Fs { return v.fs }
+
+// Unused by getLongTermPrivateKey — these satisfy VaultInterface.
+func (v *realVault) AddSecret(string, *memguard.LockedBuffer, bool) error { panic("not used") }
+func (v *realVault) GetCurrentUnlocker() (Unlocker, error)               { panic("not used") }
+func (v *realVault) CreatePassphraseUnlocker(*memguard.LockedBuffer) (*PassphraseUnlocker, error) {
+	panic("not used")
+}
+
+// createRealVault sets up a complete vault directory structure on an in-memory
+// filesystem, identical to what vault.CreateVault produces.
+func createRealVault(t *testing.T, fs afero.Fs, stateDir, name string, derivationIndex uint32) *realVault {
+	t.Helper()
+
+	vaultDir := filepath.Join(stateDir, "vaults.d", name)
+	require.NoError(t, fs.MkdirAll(filepath.Join(vaultDir, "secrets.d"), DirPerms))
+	require.NoError(t, fs.MkdirAll(filepath.Join(vaultDir, "unlockers.d"), DirPerms))
+
+	metadata := VaultMetadata{
+		CreatedAt:       time.Now(),
+		DerivationIndex: derivationIndex,
+	}
+	metaBytes, err := json.Marshal(metadata)
+	require.NoError(t, err)
+	require.NoError(t, afero.WriteFile(fs, filepath.Join(vaultDir, "vault-metadata.json"), metaBytes, FilePerms))
+
+	return &realVault{name: name, stateDir: stateDir, fs: fs}
+}
+
+func TestGetLongTermPrivateKeyUsesVaultDerivationIndex(t *testing.T) {
+	const testMnemonic = "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about"
+
+	// Derive expected keys at two different indices to prove they differ.
+	key0, err := agehd.DeriveIdentity(testMnemonic, 0)
+	require.NoError(t, err)
+	key5, err := agehd.DeriveIdentity(testMnemonic, 5)
+	require.NoError(t, err)
+	require.NotEqual(t, key0.String(), key5.String(),
+		"sanity check: different derivation indices must produce different keys")
+
+	// Build a real vault with DerivationIndex=5 on an in-memory filesystem.
+	fs := afero.NewMemMapFs()
+	vault := createRealVault(t, fs, "/state", "test-vault", 5)
+
+	t.Setenv(EnvMnemonic, testMnemonic)
+
+	result, err := getLongTermPrivateKey(fs, vault)
+	require.NoError(t, err)
+	defer result.Destroy()
+
+	assert.Equal(t, key5.String(), string(result.Bytes()),
+		"getLongTermPrivateKey should derive at vault's DerivationIndex (5)")
+	assert.NotEqual(t, key0.String(), string(result.Bytes()),
+		"getLongTermPrivateKey must not use hardcoded index 0")
+}