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/base/plugin.go

@@ -5,9 +5,12 @@ import (
 	"io/fs"
 
 	"git.golem15.com/golem15/summercms/backpack"
+	"git.golem15.com/golem15/summercms/pact"
 	"git.golem15.com/golem15/summercms/party"
 )
 
+var _ pact.HasConfig = (*Plugin)(nil)
+
 //go:embed config
 var configFS embed.FS