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/paid-ads/SKILL.md

@@ -310,6 +310,6 @@ For tracking, see also: [ga4.md](../../tools/integrations/ga4.md), [segment.md](
 
 - **ad-creative**: For generating and iterating ad headlines, descriptions, and creative at scale
 - **copywriting**: For landing page copy that converts ad traffic
-- **analytics-tracking**: For proper conversion tracking setup
-- **ab-test-setup**: For landing page testing to improve ROAS
+- **analytics**: For proper conversion tracking setup
+- **ab-testing**: For landing page testing to improve ROAS
 - **page-cro**: For optimizing post-click conversion rates