docs: backfill TOC + decision trees, fix review findings

- Add ## Contents and ## Decision Tree to all 10 existing pattern files
- Fix embed_as/1 semantics inversion in types.md (:self → :dump)
- Fix fabricated __meta__.changes reference in changesets.md
- Fix default primary key type (:integer → :id) in schemas.md
- Combine @impl subsections into single "Minimal Callback Annotation"

patterns/schemas.md

@@ -132,8 +132,8 @@ defmodule MyApp.Schema do
   defmacro __using__(_) do
     quote do
       use Ecto.Schema
-      @primary_key {:id, :integer, autogenerate: true}  # Same as the default
-      @foreign_key_type :integer
+      @primary_key {:id, :id, autogenerate: true}  # Same as the default (:id resolves to integer)
+      @foreign_key_type :id
     end
   end
 end