feat: add marketing tools registry for agent discovery

Create centralized tools/ directory with REGISTRY.md index and 29
integration guides covering analytics, SEO, CRM, payments, referral,
email, ads, automation, and commerce platforms.

Each guide includes API endpoints, authentication, common operations,
and links to relevant skills. Updated AGENTS.md and key skills
(referral-program, analytics-tracking, email-sequence, paid-ads)
with tool integration references.

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

skills/analytics-tracking/SKILL.md

@@ -285,6 +285,20 @@ dataLayer.push({
 
 ---
 
+## Tool Integrations
+
+For implementation, see the [tools registry](../../tools/REGISTRY.md). Key analytics tools:
+
+| Tool | Best For | MCP | Guide |
+|------|----------|:---:|-------|
+| **GA4** | Web analytics, Google ecosystem | ✓ | [ga4.md](../../tools/integrations/ga4.md) |
+| **Mixpanel** | Product analytics, event tracking | - | [mixpanel.md](../../tools/integrations/mixpanel.md) |
+| **Amplitude** | Product analytics, cohort analysis | - | [amplitude.md](../../tools/integrations/amplitude.md) |
+| **PostHog** | Open-source analytics, session replay | - | [posthog.md](../../tools/integrations/posthog.md) |
+| **Segment** | Customer data platform, routing | - | [segment.md](../../tools/integrations/segment.md) |
+
+---
+
 ## Related Skills
 
 - **ab-test-setup**: For experiment tracking