feat: rename 15 skills for shorter, cleaner names

Renames:
- ab-test-setup → ab-testing
- analytics-tracking → analytics
- competitor-alternatives → competitors
- email-sequence → emails
- free-tool-strategy → free-tools
- launch-strategy → launch
- onboarding-cro → onboarding
- paywall-upgrade-cro → paywalls
- popup-cro → popups
- pricing-strategy → pricing
- product-marketing-context → product-marketing
- referral-program → referrals
- schema-markup → schema
- signup-flow-cro → signup
- social-content → social

Updates all cross-references in skills, references, tools,
marketplace.json, README, AGENTS.md, and CONTRIBUTING.md.

BREAKING: Users with installed skills will need to reinstall.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

skills/churn-prevention/references/cancel-flow-patterns.md

@@ -184,7 +184,7 @@ For multi-plan products, downgrade is the strongest save:
 When the cancel reason is "switching to competitor":
 
 1. **Ask which competitor** (optional, don't force it)
-2. **Show a comparison** if you have one (see competitor-alternatives skill)
+2. **Show a comparison** if you have one (see competitors skill)
 3. **Offer a migration credit** ("We'll match their price for 3 months")
 4. **Request a feedback call** ("15 minutes to understand what we're missing")
 
@@ -226,7 +226,7 @@ from customers like you.
 | Day 60 | Address their specific cancel reason if resolved |
 | Day 90 | Final win-back with special offer |
 
-**For detailed win-back email sequences**: See the email-sequence skill.
+**For detailed win-back email sequences**: See the emails skill.
 
 ---
 
@@ -290,7 +290,7 @@ The most effective cancel flows use segmentation to show different offers to dif
 - [ ] Set up proactive intervention triggers
 - [ ] A/B test discount amounts and offer types
 - [ ] Segment flows by plan, tenure, and usage
-- [ ] Post-cancel win-back sequence (coordinate with email-sequence skill)
+- [ ] Post-cancel win-back sequence (coordinate with emails skill)
 - [ ] Cohort analysis: churn by channel, plan, tenure
 
 ---