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/referral-program/SKILL.md

@@ -231,6 +231,20 @@ They get [their reward] too.
 
 ---
 
+## Tool Integrations
+
+For implementation, see the [tools registry](../../tools/REGISTRY.md). Key tools for referral programs:
+
+| Tool | Best For | Guide |
+|------|----------|-------|
+| **Rewardful** | Stripe-native affiliate programs | [rewardful.md](../../tools/integrations/rewardful.md) |
+| **Tolt** | SaaS affiliate programs | [tolt.md](../../tools/integrations/tolt.md) |
+| **Mention Me** | Enterprise referral programs | [mention-me.md](../../tools/integrations/mention-me.md) |
+| **Dub.co** | Link tracking and attribution | [dub-co.md](../../tools/integrations/dub-co.md) |
+| **Stripe** | Payment processing (for commission tracking) | [stripe.md](../../tools/integrations/stripe.md) |
+
+---
+
 ## Related Skills
 
 - **launch-strategy**: For launching referral program effectively