Type-check and lint the macOS build from Linux (closes #50)

script/lint-darwin (make lint-darwin; run by script/check, and its
commands by the Dockerfile lint stage) runs go vet and golangci-lint
with GOOS=darwin and cgo off. Compiling cgo for macOS needs Apple's SDK,
so the three functions that call go-keychain, which is cgo there, move
to keychainunlocker_cgo.go; a macOS build without cgo gets
keychainunlocker_nocgo.go and the macse stub, whose errors name the
missing macOS build with cgo. The rest of the keychain unlocker and its
plain-Go tests are now checked; their findings are fixed without
changing behaviour, and lines over 88 columns in the unchecked files
are wrapped.

Model: opus-5-5

internal/secret/seunlocker_test.go

@@ -1,6 +1,6 @@
 //go:build darwin
-// +build darwin
 
+//nolint:testpackage // white-box test of unexported Secure Enclave helpers
 package secret
 
 import (
@@ -13,12 +13,14 @@ import (
 )
 
 func TestNewSecureEnclaveUnlocker(t *testing.T) {
+	t.Parallel()
+
 	fs := afero.NewMemMapFs()
 	dir := "/tmp/test-se-unlocker"
 	metadata := UnlockerMetadata{
-		Type:      "secure-enclave",
+		Type:      seUnlockerType,
 		CreatedAt: time.Date(2026, 1, 15, 10, 30, 0, 0, time.UTC),
-		Flags:     []string{"secure-enclave", "macos"},
+		Flags:     []string{seUnlockerType, "macos"},
 	}
 
 	unlocker := NewSecureEnclaveUnlocker(fs, dir, metadata)
@@ -35,9 +37,11 @@ func TestNewSecureEnclaveUnlocker(t *testing.T) {
 }
 
 func TestSecureEnclaveUnlockerImplementsInterface(t *testing.T) {
+	t.Parallel()
+
 	fs := afero.NewMemMapFs()
 	metadata := UnlockerMetadata{
-		Type:      "secure-enclave",
+		Type:      seUnlockerType,
 		CreatedAt: time.Now().UTC(),
 	}
 
@@ -48,9 +52,11 @@ func TestSecureEnclaveUnlockerImplementsInterface(t *testing.T) {
 }
 
 func TestSecureEnclaveUnlockerGetIDFormat(t *testing.T) {
+	t.Parallel()
+
 	fs := afero.NewMemMapFs()
 	metadata := UnlockerMetadata{
-		Type:      "secure-enclave",
+		Type:      seUnlockerType,
 		CreatedAt: time.Date(2026, 3, 10, 14, 30, 0, 0, time.UTC),
 	}
 
@@ -63,6 +69,8 @@ func TestSecureEnclaveUnlockerGetIDFormat(t *testing.T) {
 }
 
 func TestGenerateSEKeyLabel(t *testing.T) {
+	t.Parallel()
+
 	label, err := generateSEKeyLabel("test-vault")
 	require.NoError(t, err)
 
@@ -72,6 +80,8 @@ func TestGenerateSEKeyLabel(t *testing.T) {
 }
 
 func TestSecureEnclaveUnlockerGetIdentityMissingFile(t *testing.T) {
+	t.Parallel()
+
 	fs := afero.NewMemMapFs()
 	dir := "/tmp/test-se-unlocker-missing"
 
@@ -84,10 +94,12 @@ func TestSecureEnclaveUnlockerGetIdentityMissingFile(t *testing.T) {
 		"seKeyLabel": "berlin.sneak.app.secret.se.test",
 		"seKeyHash": "abc123"
 	}`
-	require.NoError(t, afero.WriteFile(fs, dir+"/unlocker-metadata.json", []byte(metadataJSON), FilePerms))
+	require.NoError(t, afero.WriteFile(
+		fs, dir+"/unlocker-metadata.json", []byte(metadataJSON), FilePerms,
+	))
 
 	metadata := UnlockerMetadata{
-		Type:      "secure-enclave",
+		Type:      seUnlockerType,
 		CreatedAt: time.Date(2026, 1, 15, 10, 30, 0, 0, time.UTC),
 	}
 
@@ -96,6 +108,6 @@ func TestSecureEnclaveUnlockerGetIdentityMissingFile(t *testing.T) {
 	// GetIdentity should fail because the encrypted longterm key file is missing
 	identity, err := unlocker.GetIdentity()
 	assert.Nil(t, identity)
-	assert.Error(t, err)
+	require.Error(t, err)
 	assert.Contains(t, err.Error(), "failed to read SE-encrypted long-term key")
 }