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/churn-prevention/SKILL.md

@@ -375,7 +375,7 @@ Test one variable at a time:
 - **Guilt-trip copy** — "Are you sure you want to abandon us?" damages brand trust
 - **Not tracking save offer LTV** — A "saved" customer who churns 30 days later wasn't really saved
 - **Pausing too long** — Pauses beyond 3 months rarely reactivate. Set limits.
-- **No post-cancel path** — Always make reactivation easy and trigger win-back emails
+- **No post-cancel path** — Make reactivation easy and trigger win-back emails, because some churned users will want to come back
 
 ---