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.go

@@ -5,12 +5,14 @@ import (
 
 	"git.golem15.com/golem15/summercms/backpack"
 	"git.golem15.com/golem15/summercms/bonfire"
+	"git.golem15.com/golem15/summercms/pact"
 	"git.golem15.com/golem15/summercms/party"
 )
 
 // Plugin is the golem15.greeter plugin. It requires golem15.hello.
 type Plugin struct {
-	app *backpack.App
+	app   *backpack.App
+	extra string
 }
 
 func (p *Plugin) ID() string         { return "golem15.greeter" }
@@ -20,6 +22,12 @@ func (p *Plugin) Register(*backpack.App) error { return nil }
 
 func (p *Plugin) Boot(app *backpack.App) error {
 	p.app = app
+	p.extra = ""
+	if app != nil && app.HasPlugin("golem15.optional") {
+		if msg, ok := app.Lookup[pact.OptionalMessage](); ok {
+			p.extra = msg.Message()
+		}
+	}
 	return nil
 }
 
@@ -36,12 +44,14 @@ func (p *Plugin) Commands() []bonfire.Command {
 				posts = p.app.Config.Int("golem15.hello.posts_per_page")
 				debug = p.app.Config.Bool("app.debug")
 			}
-			out.Printf("name=%s posts_per_page=%d debug=%t\n", name, posts, debug)
+			out.Printf("name=%s posts_per_page=%d debug=%t extra=%s\n", name, posts, debug, p.extra)
 			return nil
 		},
 	}}
 }
 
+var _ pact.HasCommands = (*Plugin)(nil)
+
 func init() {
 	party.Register(&Plugin{})
 }