feat: consolidate page-cro and form-cro into unified cro skill

- Create skills/cro/ with shared CRO analysis framework
- Move page-specific content to references/page.md
- Move form-specific content to references/form.md
- Copy experiments reference from page-cro
- Remove skills/page-cro/ and skills/form-cro/
- Update all cross-references across skills, tools, README, AGENTS.md

BREAKING: page-cro and form-cro are replaced by cro.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tools/integrations/ga4.md

@@ -123,4 +123,4 @@ POST https://www.google-analytics.com/mp/collect?measurement_id={measurement_id}
 - analytics
 - ab-testing
 - seo-audit
-- page-cro
+- cro