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>

.claude-plugin/marketplace.json

@@ -29,15 +29,14 @@
         "./skills/copy-editing",
         "./skills/copywriting",
         "./skills/customer-research",
+        "./skills/cro",
         "./skills/emails",
-        "./skills/form-cro",
         "./skills/free-tools",
         "./skills/launch",
         "./skills/lead-magnets",
         "./skills/marketing-ideas",
         "./skills/marketing-psychology",
         "./skills/onboarding",
-        "./skills/page-cro",
         "./skills/paid-ads",
         "./skills/paywalls",
         "./skills/popups",