feat(build): scaffold working admin controllers with model, AdminFS and delete (quick-261028-pq8)

- make:admin-controller edits the singular of its name (--model overrides):
  ModelName, modelClass and the YAML under models/<model>/
- NewRecord returns &models.<Model>{} when the model exists; otherwise the
  nil body stays and the command says to run make:model
- config_list.yaml offers create and delete with showCheckboxes
- registry.gen.go embeds controllers/*/*.yaml and models/*/*.yaml once they
  exist and exposes generatedAdminFS(); scaffolded plugins implement
  pact.AdminAssets through it, and a plugin without AdminFS gets a note
- blog example moves its YAML to models/post; docs and help text follow

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

@@ -20,6 +20,7 @@ var (
 	_ pact.HasCommands         = (*Plugin)(nil)
 	_ pact.HasJobs             = (*Plugin)(nil)
 	_ pact.HasAdminControllers = (*Plugin)(nil)
+	_ pact.AdminAssets         = (*Plugin)(nil)
 )
 
 //go:embed config
@@ -65,6 +66,9 @@ func (p *Plugin) AdminControllers() []pact.AdminController {
 	return generatedAdminControllers()
 }
 
+// AdminFS serves the admin YAML that registry.gen.go embeds.
+func (p *Plugin) AdminFS() fs.FS { return generatedAdminFS() }
+
 func init() {
 	party.Register(&Plugin{})
 }