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/programmatic-seo/SKILL.md

@@ -235,4 +235,4 @@ Watch for: Thin content warnings, Ranking drops, Manual actions, Crawl errors
 - **seo-audit**: For auditing programmatic pages after launch
 - **schema-markup**: For adding structured data
 - **site-architecture**: For page hierarchy, URL structure, and internal linking
-- **competitor-alternatives**: For comparison page frameworks
+- **competitors**: For comparison page frameworks