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/modules.md

@@ -2,6 +2,18 @@
 
 How modules are structured, named, and organized in Elixir core and Phoenix.
 
+## Contents
+
+1. [One Module per Concept, Nested for Sub-Concepts](#1-one-module-per-concept-nested-for-sub-concepts)
+2. [Public API at the Top, Private Functions at the Bottom](#2-public-api-at-the-top-private-functions-at-the-bottom)
+3. [`@moduledoc false` for Internal Modules](#3-moduledoc-false-for-internal-modules)
+4. [Struct Definition Conventions](#4-struct-definition-conventions)
+5. [Selective Imports in `__using__`](#5-selective-imports-in-__using__)
+6. [Alias at Module Scope for Readability](#6-alias-at-module-scope-for-readability)
+7. [Boolean-Suffixed Fields in Structs](#7-boolean-suffixed-fields-in-structs)
+
+---
+
 ## 1. One Module per Concept, Nested for Sub-Concepts
 
 **Source:** `lib/elixir/lib/` directory structure
@@ -590,4 +602,14 @@ defstruct [:user, :admin?, :count]
 
 **Why:** The `?` suffix should only mark genuine booleans. Using it on non-boolean fields creates confusion about the field's type and breaks the convention's usefulness as a type signal.
 
+## Decision Tree
+
+- If your module has grown beyond 300 lines with distinct sub-responsibilities → [One Module per Concept, Nested for Sub-Concepts](#1-one-module-per-concept-nested-for-sub-concepts)
+- If you need to decide function ordering within a module → [Public API at the Top, Private Functions at the Bottom](#2-public-api-at-the-top-private-functions-at-the-bottom)
+- If a module exists purely for internal code organization and should not appear in docs → [`@moduledoc false` for Internal Modules](#3-moduledoc-false-for-internal-modules)
+- If you need to define a struct and decide which fields are mandatory → [Struct Definition Conventions](#4-struct-definition-conventions)
+- If your `use` macro needs to set up the caller's namespace with specific functions → [Selective Imports in `__using__`](#5-selective-imports-in-__using__)
+- If multiple modules from the same parent namespace are used repeatedly → [Alias at Module Scope for Readability](#6-alias-at-module-scope-for-readability)
+- If a struct field stores a boolean value and you want self-documenting naming → [Boolean-Suffixed Fields in Structs](#7-boolean-suffixed-fields-in-structs)
+
 <!-- PATTERN_COMPLETE -->