refactor: replace rigid imperatives with reasoning-based guidance

Audit all 32 SKILL.md files against Anthropic's skill-creator writing
guidance ("why > MUST" pattern). Replaces ALWAYS/NEVER/MUST/IMPORTANT
imperatives with explanations of WHY the guidance matters.

17 edits across 14 skills:
- ad-creative: character limits reasoning, CTA headline reasoning
- seo-audit: schema detection warning softened, reasoning added
- programmatic-seo: subfolder vs subdomain reasoning
- paid-ads: exclusion list reasoning
- copywriting: honesty principle reasoning
- cold-email: follow-up value reasoning
- ai-seo: freshness signal reasoning
- churn-prevention: post-cancel path reasoning
- product-marketing-context: verbatim language reasoning
- popup-cro: close button visibility reasoning
- signup-flow-cro: label visibility reasoning
- form-cro: label visibility reasoning
- revops: fallback owner reasoning
- ab-test-setup: DON'T → Avoid

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

skills/paid-ads/SKILL.md

@@ -122,7 +122,7 @@ LI_LeadGen_CMOs-SaaS_Whitepaper_Mar24
 
 - **Lookalikes**: Base on best customers (by LTV), not all customers
 - **Retargeting**: Segment by funnel stage (visitors vs. cart abandoners)
-- **Exclusions**: Always exclude existing customers and recent converters
+- **Exclusions**: Exclude existing customers and recent converters — showing ads to people who already bought wastes spend
 
 **For detailed targeting strategies by platform**: See [references/audience-targeting.md](references/audience-targeting.md)