feat(01-02): add optional plugin services and HasPlugin

- App-scoped typed Publish/Lookup with duplicate-provider errors
- Set plugin IDs before Register so HasPlugin sees the full set
- Greeter uses pact.OptionalMessage without importing optional

internal/build/build_test.go

@@ -16,7 +16,7 @@ func TestLoadHelloManifest(t *testing.T) {
 	if m.Binary != "hello" {
 		t.Fatalf("binary = %q", m.Binary)
 	}
-	if len(m.Plugins) != 2 || m.Plugins[0].ID != "golem15.hello" || m.Plugins[1].ID != "golem15.greeter" {
+	if len(m.Plugins) != 3 || m.Plugins[0].ID != "golem15.hello" || m.Plugins[1].ID != "golem15.greeter" || m.Plugins[2].ID != "golem15.optional" {
 		t.Fatalf("plugins = %+v", m.Plugins)
 	}
 }