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/copywriting/SKILL.md

@@ -118,6 +118,8 @@ Puns and wit make copy memorable—but only if it fits the brand and doesn't und
 
 **For comprehensive headline formulas**: See [references/copy-frameworks.md](references/copy-frameworks.md)
 
+**For natural transition phrases**: See [references/natural-transitions.md](references/natural-transitions.md)
+
 **Subheadline**
 - Expands on headline
 - Adds specificity