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/ai-seo/SKILL.md

@@ -333,7 +333,7 @@ Monthly manual check:
 - Fair and balanced (AI penalizes obviously biased comparisons)
 - Specific criteria with ratings or scores
 - Updated pricing and feature data
-- Cite the competitor-alternatives skill for building these pages
+- Cite the competitors skill for building these pages
 
 ### Documentation / Help Content
 
@@ -393,6 +393,6 @@ For implementation, see the [tools registry](../../tools/REGISTRY.md).
 - **seo-audit**: For traditional technical and on-page SEO audits
 - **schema-markup**: For implementing structured data that helps AI understand your content
 - **content-strategy**: For planning what content to create
-- **competitor-alternatives**: For building comparison pages that get cited
+- **competitors**: For building comparison pages that get cited
 - **programmatic-seo**: For building SEO pages at scale
 - **copywriting**: For writing content that's both human-readable and AI-extractable