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

docs/examples/blog/controllers/posts_test.go

@@ -62,7 +62,7 @@ func TestPostsControllerDeclaration(t *testing.T) {
 	if list.ModelClass != "Post" || list.DefaultSort == nil {
 		t.Errorf("list = %+v, want modelClass Post with a default sort", list)
 	}
-	for _, name := range []string{"controllers/posts/config_form.yaml", "controllers/posts/config_list.yaml", "models/posts/fields.yaml", "models/posts/columns.yaml"} {
+	for _, name := range []string{"controllers/posts/config_form.yaml", "controllers/posts/config_list.yaml", "models/post/fields.yaml", "models/post/columns.yaml"} {
 		if _, err := fs.Stat(fsys, name); err != nil {
 			t.Errorf("AdminFS is missing %s: %v", name, err)
 		}