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/keychainunlocker_test.go

@@ -1,5 +1,4 @@
-//go:build darwin
-// +build darwin
+//go:build darwin && cgo
 
 package secret
 
@@ -35,7 +34,8 @@ func TestKeychainStoreRetrieveDelete(t *testing.T) {
 	// Test 2: Retrieve data from keychain
 	retrievedData, err := retrieveFromKeychain(testItemName)
 	require.NoError(t, err, "Failed to retrieve data from keychain")
-	assert.Equal(t, testData, string(retrievedData), "Retrieved data doesn't match stored data")
+	assert.Equal(t, testData, string(retrievedData),
+		"Retrieved data doesn't match stored data")
 
 	// Test 3: Update existing item (store again with different data)
 	newTestData := "updated-test-data-67890"
@@ -48,7 +48,8 @@ func TestKeychainStoreRetrieveDelete(t *testing.T) {
 	// Verify updated data
 	retrievedData, err = retrieveFromKeychain(testItemName)
 	require.NoError(t, err, "Failed to retrieve updated data from keychain")
-	assert.Equal(t, newTestData, string(retrievedData), "Retrieved data doesn't match updated data")
+	assert.Equal(t, newTestData, string(retrievedData),
+		"Retrieved data doesn't match updated data")
 
 	// Test 4: Delete from keychain
 	err = deleteFromKeychain(testItemName)
@@ -93,7 +94,8 @@ func TestKeychainInvalidItemName(t *testing.T) {
 	for _, name := range invalidNames {
 		err := storeInKeychain(name, testData)
 		assert.Error(t, err, "Expected error for invalid name: %s", name)
-		assert.Contains(t, err.Error(), "invalid keychain item name", "Error should mention invalid name for: %s", name)
+		assert.Contains(t, err.Error(), "invalid keychain item name",
+			"Error should mention invalid name for: %s", name)
 	}
 
 	// Test valid names (should not error on validation)
@@ -180,5 +182,6 @@ func TestDeleteNonExistentKeychainItem(t *testing.T) {
 	// This is important for cleaning up unlocker directories when the keychain item
 	// has already been removed (e.g., manually by user, or on a different machine)
 	err := deleteFromKeychain(testItemName)
-	assert.NoError(t, err, "Deleting non-existent keychain item should not return an error")
+	assert.NoError(t, err,
+		"Deleting non-existent keychain item should not return an error")
 }