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/customer-research/SKILL.md

@@ -262,7 +262,7 @@ Don't ask all five at once — lead with #1 and #2, then follow up as needed.
 |-----------------|-------|
 | Writing copy informed by the research | `copywriting` |
 | Optimizing a page using VOC insights | `page-cro` |
-| Building a competitor comparison page | `competitor-alternatives` |
+| Building a competitor comparison page | `competitors` |
 | Creating a churn prevention strategy from churn research | `churn-prevention` |
 | Planning paid ads informed by research | `paid-ads` |
 | Writing cold email using research on pain/trigger | `cold-email` |