feat(build): mark scaffolded files with //summer:make and pass the app to generated commands (quick-261028-pq8)

- model, migration, command, job and admin-controller stubs start with
  //summer:make <kind>; registry.gen.go keeps its generated-code header
- the registry scan accepts the directive and the older header
- command functions may take one *backpack.App; generatedCommands takes a
  variadic app, so plugin.go files calling generatedCommands() still compile
- scaffolded plugins keep the app from Boot and return generatedCommands(p.app)
- make:command notes the nil app for an older plugin.go
- docs, help text and the blog example follow

internal/build/testdata/new/plugins__golem15__demo__plugin.go.golden

@@ -32,13 +32,20 @@ var langFS embed.FS
 var mailFS embed.FS
 
 // Plugin is the golem15.demo plugin.
-type Plugin struct{}
+type Plugin struct {
+	// app is the application, kept from Boot for the generated commands.
+	app *backpack.App
+}
 
 func (p *Plugin) ID() string         { return "golem15.demo" }
 func (p *Plugin) Requires() []string { return nil }
 
 func (p *Plugin) Register(*backpack.App) error { return nil }
-func (p *Plugin) Boot(*backpack.App) error     { return nil }
+
+func (p *Plugin) Boot(app *backpack.App) error {
+	p.app = app
+	return nil
+}
 
 func (p *Plugin) ConfigFS() fs.FS { return configFS }
 func (p *Plugin) LangFS() fs.FS   { return langFS }
@@ -49,8 +56,11 @@ func (p *Plugin) MailLayouts() map[string]string { return nil }
 
 func (p *Plugin) Models() []any                       { return generatedModels() }
 func (p *Plugin) Migrations() []*gormigrate.Migration { return generatedMigrations() }
-func (p *Plugin) Commands() []bonfire.Command         { return generatedCommands() }
-func (p *Plugin) Jobs() []pact.Job                    { return generatedJobs() }
+
+// Commands is collected after Boot, so p.app is set.
+func (p *Plugin) Commands() []bonfire.Command { return generatedCommands(p.app) }
+
+func (p *Plugin) Jobs() []pact.Job { return generatedJobs() }
 func (p *Plugin) AdminControllers() []pact.AdminController {
 	return generatedAdminControllers()
 }