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

examples/hello/plugins/greeter/plugin_test.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"fmt"
 	"io"
+	"os"
 	"strings"
 	"testing"
 
@@ -61,6 +62,16 @@ func TestBootUsesPublishedOptionalService(t *testing.T) {
 	}
 }
 
+func TestGreeterSourceDoesNotImportOptional(t *testing.T) {
+	body, err := os.ReadFile("plugin.go")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(string(body), "plugins/optional") {
+		t.Fatal("greeter must not import the optional plugin module")
+	}
+}
+
 func runHelloCmd(t *testing.T, p *Plugin) string {
 	t.Helper()
 	cmds := p.Commands()