fix: add table of contents to all reference files

Per Anthropic's Agent Skills best practices, reference files over
  100 lines should include a TOC so Claude can see full scope when
  previewing with partial reads. Added ## Contents section to all
  32 reference files.

skills/analytics-tracking/references/gtm-implementation.md

@@ -2,6 +2,16 @@
 
 Detailed guide for implementing tracking via Google Tag Manager.
 
+## Contents
+- Container Structure (tags, triggers, variables)
+- Naming Conventions
+- Data Layer Patterns
+- Common Tag Configurations (GA4 configuration tag, GA4 event tag, Facebook pixel)
+- Preview and Debug
+- Workspaces and Versioning
+- Consent Management
+- Advanced Patterns (tag sequencing, exception handling, custom JavaScript variables)
+
 ## Container Structure
 
 ### Tags