Validate secret name in GetSecretVersion to prevent path traversal

Add isValidSecretName() check at the top of GetSecretVersion(), matching
the existing validation in AddSecret(). Without this, crafted secret names
containing path traversal sequences (e.g. '../../../etc/passwd') could be
used to read files outside the vault directory.

Add regression tests for both GetSecretVersion and GetSecret.

Closes #13

internal/vault/path_traversal_test.go

@@ -0,0 +1,66 @@
+package vault
+
+import (
+	"testing"
+
+	"git.eeqj.de/sneak/secret/internal/secret"
+	"github.com/awnumar/memguard"
+	"github.com/spf13/afero"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestGetSecretVersionRejectsPathTraversal verifies that GetSecretVersion
+// validates the secret name and rejects path traversal attempts.
+// This is a regression test for https://git.eeqj.de/sneak/secret/issues/13
+func TestGetSecretVersionRejectsPathTraversal(t *testing.T) {
+	testMnemonic := "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about"
+	t.Setenv(secret.EnvMnemonic, testMnemonic)
+	t.Setenv(secret.EnvUnlockPassphrase, "test-passphrase")
+
+	fs := afero.NewMemMapFs()
+	stateDir := "/test/state"
+
+	vlt, err := CreateVault(fs, stateDir, "test-vault")
+	require.NoError(t, err)
+
+	// Add a legitimate secret so the vault is set up
+	value := memguard.NewBufferFromBytes([]byte("legitimate-secret"))
+	err = vlt.AddSecret("legit", value, false)
+	require.NoError(t, err)
+
+	// These names contain path traversal and should be rejected
+	maliciousNames := []string{
+		"../../../etc/passwd",
+		"..%2f..%2fetc/passwd",
+		".secret",
+		"../sibling-vault/secrets.d/target",
+	}
+
+	for _, name := range maliciousNames {
+		t.Run(name, func(t *testing.T) {
+			_, err := vlt.GetSecretVersion(name, "")
+			assert.Error(t, err, "GetSecretVersion should reject malicious name: %s", name)
+			assert.Contains(t, err.Error(), "invalid secret name",
+				"error should indicate invalid name for: %s", name)
+		})
+	}
+}
+
+// TestGetSecretRejectsPathTraversal verifies GetSecret (which calls GetSecretVersion)
+// also rejects path traversal names.
+func TestGetSecretRejectsPathTraversal(t *testing.T) {
+	testMnemonic := "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about"
+	t.Setenv(secret.EnvMnemonic, testMnemonic)
+	t.Setenv(secret.EnvUnlockPassphrase, "test-passphrase")
+
+	fs := afero.NewMemMapFs()
+	stateDir := "/test/state"
+
+	vlt, err := CreateVault(fs, stateDir, "test-vault")
+	require.NoError(t, err)
+
+	_, err = vlt.GetSecret("../../../etc/passwd")
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "invalid secret name")
+}