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/copywriting/SKILL.md

@@ -67,7 +67,7 @@ Each section should advance one argument. Build a logical flow down the page.
 3. **Active over passive** — "We generate reports" not "Reports are generated"
 4. **Confident over qualified** — Remove "almost," "very," "really"
 5. **Show over tell** — Describe the outcome instead of using adverbs
-6. **Honest over sensational** — Never fabricate statistics or testimonials
+6. **Honest over sensational** — Fabricated statistics or testimonials erode trust and create legal liability
 
 ### Quick Quality Check