feat: add skill version tracking system

- Add version: 1.0.0 to all 25 skill YAML frontmatters
- Create VERSIONS.md manifest listing all skill versions
- Add update check instructions to AGENTS.md

This enables users to be notified of skill updates and easily
pull the latest changes when 2+ skills are updated or there is
a major version bump.

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

VERSIONS.md

@@ -0,0 +1,37 @@
+# Marketing Skills Versions
+
+Current versions of all skills. Agents can compare against local versions to check for updates.
+
+| Skill | Version | Last Updated |
+|-------|---------|--------------|
+| ab-test-setup | 1.0.0 | 2026-01-27 |
+| analytics-tracking | 1.0.0 | 2026-01-27 |
+| competitor-alternatives | 1.0.0 | 2026-01-27 |
+| content-strategy | 1.0.0 | 2026-01-27 |
+| copy-editing | 1.0.0 | 2026-01-27 |
+| copywriting | 1.0.0 | 2026-01-27 |
+| email-sequence | 1.0.0 | 2026-01-27 |
+| form-cro | 1.0.0 | 2026-01-27 |
+| free-tool-strategy | 1.0.0 | 2026-01-27 |
+| launch-strategy | 1.0.0 | 2026-01-27 |
+| marketing-ideas | 1.0.0 | 2026-01-27 |
+| marketing-psychology | 1.0.0 | 2026-01-27 |
+| onboarding-cro | 1.0.0 | 2026-01-27 |
+| page-cro | 1.0.0 | 2026-01-27 |
+| paid-ads | 1.0.0 | 2026-01-27 |
+| paywall-upgrade-cro | 1.0.0 | 2026-01-27 |
+| popup-cro | 1.0.0 | 2026-01-27 |
+| pricing-strategy | 1.0.0 | 2026-01-27 |
+| product-marketing-context | 1.0.0 | 2026-01-27 |
+| programmatic-seo | 1.0.0 | 2026-01-27 |
+| referral-program | 1.0.0 | 2026-01-27 |
+| schema-markup | 1.0.0 | 2026-01-27 |
+| seo-audit | 1.0.0 | 2026-01-27 |
+| signup-flow-cro | 1.0.0 | 2026-01-27 |
+| social-content | 1.0.0 | 2026-01-27 |
+
+## Recent Changes
+
+### 2026-01-27
+- Initial version tracking added
+- Added tools registry with 29 integration guides