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>

tools/integrations/customer-io.md

@@ -0,0 +1,187 @@
+# Customer.io
+
+Behavior-based messaging platform for email, push, SMS, and in-app.
+
+## Capabilities
+
+| Integration | Available | Notes |
+|-------------|-----------|-------|
+| API | ✓ | Track API, App API, Journeys API |
+| MCP | - | Not available |
+| CLI | - | Not available |
+| SDK | ✓ | JavaScript, iOS, Android, Ruby, Python |
+
+## Authentication
+
+- **Track API**: Site ID + API Key (Basic auth)
+- **App API**: Bearer token
+- **Header**: `Authorization: Basic {base64(site_id:api_key)}`
+
+## Common Agent Operations
+
+### Identify customer
+
+```bash
+PUT https://track.customer.io/api/v1/customers/{customer_id}
+
+Authorization: Basic {base64(site_id:api_key)}
+
+{
+  "email": "user@example.com",
+  "created_at": 1705312800,
+  "first_name": "John",
+  "plan": "pro"
+}
+```
+
+### Track event
+
+```bash
+POST https://track.customer.io/api/v1/customers/{customer_id}/events
+
+Authorization: Basic {base64(site_id:api_key)}
+
+{
+  "name": "purchase",
+  "data": {
+    "product": "Pro Plan",
+    "amount": 99
+  }
+}
+```
+
+### Track anonymous event
+
+```bash
+POST https://track.customer.io/api/v1/events
+
+Authorization: Basic {base64(site_id:api_key)}
+
+{
+  "name": "page_viewed",
+  "data": {
+    "page": "/pricing"
+  },
+  "anonymous_id": "anon_123"
+}
+```
+
+### Delete customer
+
+```bash
+DELETE https://track.customer.io/api/v1/customers/{customer_id}
+
+Authorization: Basic {base64(site_id:api_key)}
+```
+
+### Get customer (App API)
+
+```bash
+GET https://api.customer.io/v1/customers/{customer_id}/attributes
+
+Authorization: Bearer {app_api_key}
+```
+
+### List campaigns
+
+```bash
+GET https://api.customer.io/v1/campaigns
+
+Authorization: Bearer {app_api_key}
+```
+
+### Get campaign metrics
+
+```bash
+GET https://api.customer.io/v1/campaigns/{campaign_id}/metrics
+
+Authorization: Bearer {app_api_key}
+```
+
+### Trigger broadcast
+
+```bash
+POST https://api.customer.io/v1/campaigns/{campaign_id}/triggers
+
+Authorization: Bearer {app_api_key}
+
+{
+  "emails": ["user@example.com"],
+  "data": {
+    "coupon_code": "SAVE20"
+  }
+}
+```
+
+### Send transactional email
+
+```bash
+POST https://api.customer.io/v1/send/email
+
+Authorization: Bearer {app_api_key}
+
+{
+  "transactional_message_id": "1",
+  "to": "user@example.com",
+  "identifiers": {
+    "id": "user_123"
+  },
+  "message_data": {
+    "order_id": "ORD-456"
+  }
+}
+```
+
+## JavaScript SDK
+
+```javascript
+// Initialize
+_cio.identify({
+  id: 'user_123',
+  email: 'user@example.com',
+  created_at: 1705312800,
+  plan: 'pro'
+});
+
+// Track event
+_cio.track('purchase', {
+  product: 'Pro Plan',
+  amount: 99
+});
+
+// Track page view
+_cio.page();
+```
+
+## Key Concepts
+
+- **People** - Customers and leads
+- **Segments** - Dynamic groups based on attributes/behavior
+- **Campaigns** - Automated message sequences
+- **Broadcasts** - One-time sends
+- **Transactional** - Triggered messages
+
+## Attribute Types
+
+- Standard: `email`, `created_at`, `unsubscribed`
+- Custom: Any key you define
+- Computed: Aggregations from events
+
+## When to Use
+
+- Behavior-based email automation
+- Multi-channel messaging (email, push, SMS)
+- Onboarding sequences
+- Re-engagement campaigns
+- Transactional messages
+
+## Rate Limits
+
+- Track API: 100 requests/second
+- App API: 10 requests/second
+
+## Relevant Skills
+
+- email-sequence
+- onboarding-cro
+- analytics-tracking