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/ad-creative/SKILL.md

@@ -64,7 +64,7 @@ Pull performance data → Identify winning patterns → Generate new variations
 
 ## Platform Specs
 
-**Always enforce these limits.** Never deliver creative that exceeds platform character limits.
+Platforms reject or truncate creative that exceeds these limits, so verify every piece of copy fits before delivering.
 
 ### Google Ads (Responsive Search Ads)
 
@@ -320,7 +320,7 @@ For large-scale creative production (Anthropic's growth team generates 100+ vari
 - **Writing headlines that only work together** — RSA headlines get combined randomly
 - **Ignoring character limits** — Platforms truncate without warning
 - **All variations sound the same** — Vary angles, not just word choice
-- **No CTA headlines** — Always include action-oriented headlines
+- **No CTA headlines** — RSAs need action-oriented headlines to drive clicks; include at least 2-3
 - **Generic descriptions** — "Learn more about our solution" wastes the slot
 - **Iterating without data** — Gut feelings are less reliable than metrics
 - **Testing too many things at once** — Change one variable per test cycle