Add experiment ideas sections to all CRO skills

Adds curated A/B test experiments from SaaS CRO playbook to:
- page-cro: homepage, pricing, demo, resource page experiments
- signup-flow-cro: form design, copy, trial, post-submit experiments
- onboarding-cro: flow simplification, guided experience, personalization
- form-cro: structure, copy, mobile, type-specific experiments
- popup-cro: placement, triggers, messaging, personalization
- paywall-upgrade-cro: triggers, design, pricing, trials, personalization

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

skills/signup-flow-cro.md

@@ -257,6 +257,85 @@ Organized by:
 
 ---
 
+## Experiment Ideas
+
+### Form Design Experiments
+
+**Layout & Structure**
+- Single-step vs. multi-step signup flow
+- Multi-step with progress bar vs. without
+- 1-column vs. 2-column field layout
+- Form embedded on page vs. separate signup page
+- Horizontal vs. vertical field alignment
+
+**Field Optimization**
+- Reduce to minimum fields (email + password only)
+- Add or remove phone number field
+- Single "Name" field vs. "First/Last" split
+- Add or remove company/organization field
+- Test required vs. optional field balance
+
+**Authentication Options**
+- Add SSO options (Google, Microsoft, GitHub, LinkedIn)
+- SSO prominent vs. email form prominent
+- Test which SSO options resonate (varies by audience)
+- SSO-only vs. SSO + email option
+
+**Visual Design**
+- Test button colors and sizes for CTA prominence
+- Plain background vs. product-related visuals
+- Test form container styling (card vs. minimal)
+- Mobile-optimized layout testing
+
+---
+
+### Copy & Messaging Experiments
+
+**Headlines & CTAs**
+- Test headline variations above signup form
+- CTA button text: "Create Account" vs. "Start Free Trial" vs. "Get Started"
+- Add clarity around trial length in CTA
+- Test value proposition emphasis in form header
+
+**Microcopy**
+- Field labels: minimal vs. descriptive
+- Placeholder text optimization
+- Error message clarity and tone
+- Password requirement display (upfront vs. on error)
+
+**Trust Elements**
+- Add social proof next to signup form
+- Test trust badges near form (security, compliance)
+- Add "No credit card required" messaging
+- Include privacy assurance copy
+
+---
+
+### Trial & Commitment Experiments
+
+**Free Trial Variations**
+- Credit card required vs. not required for trial
+- Test trial length impact (7 vs. 14 vs. 30 days)
+- Freemium vs. free trial model
+- Trial with limited features vs. full access
+
+**Friction Points**
+- Email verification required vs. delayed vs. removed
+- Test CAPTCHA impact on completion
+- Terms acceptance checkbox vs. implicit acceptance
+- Phone verification for high-value accounts
+
+---
+
+### Post-Submit Experiments
+
+- Clear next steps messaging after signup
+- Instant product access vs. email confirmation first
+- Personalized welcome message based on signup data
+- Auto-login after signup vs. require login
+
+---
+
 ## Questions to Ask
 
 If you need more context: