Add competitor-alternatives skill, update tooling, add attribution

- New skill: competitor-alternatives for vs pages and alternative pages
  - Four formats: singular alt, plural alts, you vs X, X vs Y
  - Modular competitor data architecture
  - Section templates beyond feature tables

- ab-test-setup: Replace Google Optimize with PostHog as primary tool

- README: Add author attribution with links to corey.co and Conversion Factory

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

skills/ab-test-setup.md

@@ -259,7 +259,7 @@ Variant (B):
 
 ### Client-Side Testing
 
-**Tools**: Optimizely, VWO, Google Optimize (sunset), custom
+**Tools**: PostHog, Optimizely, VWO, custom
 
 **How it works**:
 - JavaScript modifies page after load
@@ -273,7 +273,7 @@ Variant (B):
 
 ### Server-Side Testing
 
-**Tools**: LaunchDarkly, Split, custom
+**Tools**: PostHog, LaunchDarkly, Split, custom
 
 **How it works**:
 - Variant determined before page renders