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/copy-editing/references/plain-english-alternatives.md

@@ -6,6 +6,24 @@ Source: Plain English Campaign A-Z of Alternative Words (2001), Australian Gover
 
 ---
 
+## Contents
+- A
+- B
+- C
+- D
+- E
+- F
+- G-H
+- I
+- L-M
+- N-O
+- P
+- R
+- S
+- T-U
+- V-Z
+- Phrases to Remove Entirely
+
 ## A
 
 | Complex | Plain Alternative |