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.gen.go

@@ -5,10 +5,12 @@ package main
 import (
 	_ "git.golem15.com/golem15/summercms/examples/hello/plugins/base"
 	_ "git.golem15.com/golem15/summercms/examples/hello/plugins/greeter"
+	_ "git.golem15.com/golem15/summercms/examples/hello/plugins/optional"
 )
 
 // PluginIDs is the ordered plugin list from summer.yaml.
 var PluginIDs = []string{
 	"golem15.hello",
 	"golem15.greeter",
+	"golem15.optional",
 }