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

modules/cabana/README.md

@@ -192,7 +192,7 @@ Plugin CSS (declared through `pact.AdminClientAssets`) and any widget shadow DOM
 
 ## Usage
 
-A plugin exposes an admin controller and embeds its YAML. `summer make:admin-controller` scaffolds the controller type and its four YAML files:
+A plugin exposes an admin controller and embeds its YAML. `summer make:admin-controller` scaffolds the controller type and its four YAML files, names the model after the singular of the controller (or `--model`) and lists the controller in the plugin's generated `registry.gen.go`, which also embeds the YAML for a scaffolded plugin's `AdminFS`. Written by hand, the same pieces look like this:
 
 ```go
 package blog