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/ad-creative/SKILL.md

@@ -357,6 +357,6 @@ node tools/clis/google-ads.js reports get --type ad_performance --date-range las
 
 - **paid-ads**: For campaign strategy, targeting, budgets, and optimization
 - **copywriting**: For landing page copy (where ad traffic lands)
-- **ab-test-setup**: For structuring creative tests with statistical rigor
+- **ab-testing**: For structuring creative tests with statistical rigor
 - **marketing-psychology**: For psychological principles behind high-performing creative
 - **copy-editing**: For polishing ad copy before launch