Add valuable reference content from PR #10

Cherry-picked the genuinely unique content from PR #10 and integrated
into existing skills:

seo-audit/references:
- ai-writing-detection.md: AI writing patterns to avoid (em dashes,
  overused phrases, filler words)
- aeo-geo-patterns.md: Answer Engine & Generative Engine Optimization
  content patterns

copy-editing/references:
- plain-english-alternatives.md: Complex to simple word replacements

copywriting/references:
- natural-transitions.md: Transitional phrases for better content flow

Updated SKILL.md files to link to new references.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

skills/copy-editing/SKILL.md

@@ -411,6 +411,12 @@ This iterative process ensures each edit doesn't create new problems while respe
 
 ---
 
+## References
+
+- [Plain English Alternatives](references/plain-english-alternatives.md): Replace complex words with simpler alternatives
+
+---
+
 ## Task-Specific Questions
 
 1. What's the goal of this copy? (Awareness, conversion, retention)