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/email-sequence/SKILL.md

@@ -283,6 +283,20 @@ What to measure and benchmarks
 
 ---
 
+## Tool Integrations
+
+For implementation, see the [tools registry](../../tools/REGISTRY.md). Key email tools:
+
+| Tool | Best For | MCP | Guide |
+|------|----------|:---:|-------|
+| **Customer.io** | Behavior-based automation | - | [customer-io.md](../../tools/integrations/customer-io.md) |
+| **Mailchimp** | SMB email marketing | ✓ | [mailchimp.md](../../tools/integrations/mailchimp.md) |
+| **Resend** | Developer-friendly transactional | ✓ | [resend.md](../../tools/integrations/resend.md) |
+| **SendGrid** | Transactional email at scale | - | [sendgrid.md](../../tools/integrations/sendgrid.md) |
+| **Kit** | Creator/newsletter focused | - | [kit.md](../../tools/integrations/kit.md) |
+
+---
+
 ## Related Skills
 
 - **onboarding-cro**: For in-app onboarding (email supports this)