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/ai-seo/SKILL.md

@@ -353,7 +353,7 @@ Monthly manual check:
 - **Ignoring AI search entirely** — ~45% of Google searches now show AI Overviews, and ChatGPT/Perplexity are growing fast
 - **Treating AI SEO as separate from SEO** — Good traditional SEO is the foundation; AI SEO adds structure and authority on top
 - **Writing for AI, not humans** — If content reads like it was written to game an algorithm, it won't get cited or convert
-- **No freshness signals** — Undated content loses to dated content. Always show when content was last updated
+- **No freshness signals** — Undated content loses to dated content because AI systems weight recency heavily. Show when content was last updated
 - **Gating all content** — AI can't access gated content. Keep your most authoritative content open
 - **Ignoring third-party presence** — You may get more AI citations from a Wikipedia mention than from your own blog
 - **No structured data** — Schema markup gives AI systems structured context about your content