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

pact/capabilities.go

@@ -18,7 +18,16 @@ type HasConfig interface {
 	ConfigFS() fs.FS
 }
 
-// Reserved KERN-03 capability names, declared when their first consumers exist:
+// OptionalMessage is a service an optional plugin may publish so other
+// plugins can integrate without importing that plugin's package.
+type OptionalMessage interface {
+	Message() string
+}
+
+// Future KERN-03 capability families are type-asserted when their first
+// consumer packages exist. Method payloads are not declared here so pact
+// does not import surf, lagoon, festival, or other packages that Phase 1
+// does not yet ship:
 //
 //	HasModels
 //	HasMigrations
@@ -32,3 +41,6 @@ type HasConfig interface {
 //	HasSchedule
 //	HasMailTemplates
 //	HasLang
+//
+// Phase 1 type-asserts only HasConfig (party, before Register) and
+// HasCommands (generated app main, after Boot).