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/models/post/columns.yaml

@@ -0,0 +1,10 @@
+columns:
+    title:
+        label: acme.blog::lang.posts.title_field
+        searchable: true
+    slug:
+        label: acme.blog::lang.posts.slug
+        searchable: true
+    published_at:
+        label: acme.blog::lang.posts.published_at
+        type: datetime

docs/examples/blog/models/post/fields.yaml

@@ -0,0 +1,15 @@
+fields:
+    title:
+        label: acme.blog::lang.posts.title_field
+        type: text
+        span: left
+        required: true
+    slug:
+        label: acme.blog::lang.posts.slug
+        type: text
+        span: right
+        required: true
+    body:
+        label: acme.blog::lang.posts.body
+        type: textarea
+        size: large