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>

CONTRIBUTING.md

@@ -33,7 +33,7 @@ Optional frontmatter fields: `license` (default: MIT), `metadata` (author, versi
 
 ### 3. Follow the naming conventions
 
-- **Directory name**: lowercase, hyphens only (e.g., `email-sequence`)
+- **Directory name**: lowercase, hyphens only (e.g., `emails`)
 - **Name field**: must match directory name exactly
 - **Description**: 1-1024 characters, include trigger phrases