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/referral-program/references/program-examples.md

@@ -2,6 +2,15 @@
 
 Real-world examples of successful referral programs.
 
+## Contents
+- Dropbox (Classic)
+- Uber/Lyft
+- Morning Brew
+- Notion
+- Incentive Types Comparison
+- Incentive Sizing Framework
+- Viral Coefficient & Metrics (Key Metrics, Calculating Referral Program ROI)
+
 ## Dropbox (Classic)
 
 **Program:** Give 500MB storage, get 500MB storage