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,
and keychainunlocker.go uses go-keychain, which is cgo there, so it and
its tests are now built only with cgo on macOS, like internal/macse;
their stubs serve a macOS build without cgo. checkMacOSAvailable moves
to seunlocker_darwin.go. The findings in the newly checked files are
fixed, and lines over 88 columns in the unchecked ones are wrapped.

Model: opus-5-5

internal/macse/macse_test.go

@@ -1,5 +1,4 @@
-//go:build darwin
-// +build darwin
+//go:build darwin && cgo
 
 package macse
 
@@ -45,7 +44,8 @@ func TestCreateAndDeleteKey(t *testing.T) {
 
 	// Verify valid uncompressed P-256 public key
 	if len(pubKey) != p256UncompressedKeySize {
-		t.Fatalf("expected public key length %d, got %d", p256UncompressedKeySize, len(pubKey))
+		t.Fatalf("expected public key length %d, got %d",
+			p256UncompressedKeySize, len(pubKey))
 	}
 
 	if pubKey[0] != 0x04 {
@@ -83,7 +83,8 @@ func TestEncryptDecryptRoundTrip(t *testing.T) {
 	}()
 
 	// Test data simulating an age private key
-	plaintext := []byte("AGE-SECRET-KEY-1QQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQ")
+	plaintext := []byte("AGE-SECRET-KEY-1" +
+		"QQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQ")
 
 	// Encrypt
 	ciphertext, err := Encrypt(testKeyLabel, plaintext)