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/ga4.md

@@ -0,0 +1,126 @@
+# Google Analytics 4 (GA4)
+
+Web analytics platform for tracking user behavior, conversions, and marketing performance.
+
+## Capabilities
+
+| Integration | Available | Notes |
+|-------------|-----------|-------|
+| API | ✓ | Data API for reports, Admin API for configuration |
+| MCP | ✓ | Available via Google Analytics MCP server |
+| CLI | - | Use gcloud for some operations |
+| SDK | ✓ | gtag.js, Google Analytics SDK for mobile |
+
+## Authentication
+
+- **Type**: OAuth 2.0 or Service Account
+- **Scopes**: `https://www.googleapis.com/auth/analytics.readonly` (read), `https://www.googleapis.com/auth/analytics.edit` (write)
+- **Setup**: Create credentials in Google Cloud Console
+
+## Common Agent Operations
+
+### Run a report (Data API)
+
+```bash
+POST https://analyticsdata.googleapis.com/v1beta/properties/{property_id}:runReport
+
+{
+  "dateRanges": [{"startDate": "30daysAgo", "endDate": "today"}],
+  "dimensions": [{"name": "sessionSource"}],
+  "metrics": [{"name": "sessions"}, {"name": "conversions"}]
+}
+```
+
+### Get real-time data
+
+```bash
+POST https://analyticsdata.googleapis.com/v1beta/properties/{property_id}:runRealtimeReport
+
+{
+  "dimensions": [{"name": "country"}],
+  "metrics": [{"name": "activeUsers"}]
+}
+```
+
+### List conversion events
+
+```bash
+GET https://analyticsadmin.googleapis.com/v1beta/properties/{property_id}/conversionEvents
+```
+
+### Create a conversion event
+
+```bash
+POST https://analyticsadmin.googleapis.com/v1beta/properties/{property_id}/conversionEvents
+
+{
+  "eventName": "purchase"
+}
+```
+
+## Client-Side Tracking
+
+### Send custom event (gtag.js)
+
+```javascript
+gtag('event', 'signup_completed', {
+  'method': 'email',
+  'plan': 'free'
+});
+```
+
+### Send event via Measurement Protocol
+
+```bash
+POST https://www.google-analytics.com/mp/collect?measurement_id={measurement_id}&api_secret={api_secret}
+
+{
+  "client_id": "client_123",
+  "events": [{
+    "name": "purchase",
+    "params": {
+      "value": 99.99,
+      "currency": "USD"
+    }
+  }]
+}
+```
+
+## Key Dimensions & Metrics
+
+### Common Dimensions
+- `sessionSource` - Traffic source
+- `sessionMedium` - Traffic medium
+- `sessionCampaignName` - Campaign name
+- `landingPage` - Entry page
+- `deviceCategory` - Device type
+- `country` - User country
+
+### Common Metrics
+- `sessions` - Total sessions
+- `activeUsers` - Active users
+- `newUsers` - New users
+- `conversions` - Conversion events
+- `engagementRate` - Engaged sessions rate
+- `averageSessionDuration` - Session duration
+
+## When to Use
+
+- Tracking website traffic and user behavior
+- Measuring marketing campaign performance
+- Setting up conversion tracking
+- Analyzing user journeys and funnels
+- Attribution modeling
+
+## Rate Limits
+
+- Data API: 10 requests per second per property
+- Admin API: Varies by endpoint
+- Measurement Protocol: 1M hits/day for free tier
+
+## Relevant Skills
+
+- analytics-tracking
+- ab-test-setup
+- seo-audit
+- page-cro