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

party/registry.go

@@ -98,6 +98,12 @@ func activate(registered []Plugin, app *backpack.App, ids []string) ([]Plugin, e
 		return nil, err
 	}
 
+	orderedIDs := make([]string, len(ordered))
+	for i, p := range ordered {
+		orderedIDs[i] = p.ID()
+	}
+	app.SetPlugins(orderedIDs)
+
 	for _, p := range ordered {
 		if hc, ok := p.(pact.HasConfig); ok && app.Config != nil {
 			if err := app.Config.MergePlugin(p.ID(), hc.ConfigFS()); err != nil {