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>

skills/signup/evals/evals.json

@@ -33,7 +33,7 @@
     {
       "id": 3,
       "prompt": "our signup form is just email and password but we still only get 35% of visitors to complete it. what else could be wrong?",
-      "expected_output": "Should trigger on casual phrasing. Should investigate beyond just form fields since the form is already minimal. Should apply trust and friction reduction guidance: is there a 'No credit card required' message? Privacy assurance? Testimonial near the form? Should check form-level issues: error handling, password requirements clarity, submit button copy. Should also look at pre-form factors: is the value proposition clear? Is the page optimized? (cross-reference page-cro). Should provide diagnostic checklist and recommendations.",
+      "expected_output": "Should trigger on casual phrasing. Should investigate beyond just form fields since the form is already minimal. Should apply trust and friction reduction guidance: is there a 'No credit card required' message? Privacy assurance? Testimonial near the form? Should check form-level issues: error handling, password requirements clarity, submit button copy. Should also look at pre-form factors: is the value proposition clear? Is the page optimized? (cross-reference cro). Should provide diagnostic checklist and recommendations.",
       "assertions": [
         "Triggers on casual phrasing",
         "Investigates beyond form fields",