feat(01-01): boot hello app and dispatch greeter:hello

- Add party plugin registry with Register-before-Boot activation
- Add backpack, compass, bonfire, and pact capability interfaces
- Add examples/hello with two compiled plugin modules

backpack/app.go

@@ -0,0 +1,13 @@
+package backpack
+
+import "git.golem15.com/golem15/summercms/compass"
+
+// App is the per-instance application container. It must not import party.
+type App struct {
+	Config *compass.Config
+}
+
+// New returns an App that owns cfg.
+func New(cfg *compass.Config) *App {
+	return &App{Config: cfg}
+}