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/ahrefs.md

@@ -0,0 +1,142 @@
+# Ahrefs
+
+SEO toolset for backlink analysis, keyword research, and competitive research.
+
+## Capabilities
+
+| Integration | Available | Notes |
+|-------------|-----------|-------|
+| API | ✓ | REST API for Site Explorer, Keywords Explorer |
+| MCP | - | Not available |
+| CLI | - | Not available |
+| SDK | - | API-only |
+
+## Authentication
+
+- **Type**: API Token
+- **Header**: `Authorization: Bearer {api_token}`
+- **Get token**: Account Settings > API in Ahrefs dashboard
+
+## Common Agent Operations
+
+### Domain rating
+
+```bash
+GET https://api.ahrefs.com/v3/site-explorer/domain-rating?target=example.com
+
+Authorization: Bearer {api_token}
+```
+
+### Backlinks overview
+
+```bash
+GET https://api.ahrefs.com/v3/site-explorer/backlinks-stats?target=example.com&mode=domain
+
+Authorization: Bearer {api_token}
+```
+
+### Referring domains
+
+```bash
+GET https://api.ahrefs.com/v3/site-explorer/refdomains?target=example.com&mode=domain&limit=100
+
+Authorization: Bearer {api_token}
+```
+
+### Backlinks list
+
+```bash
+GET https://api.ahrefs.com/v3/site-explorer/backlinks?target=example.com&mode=domain&limit=100
+
+Authorization: Bearer {api_token}
+```
+
+### Organic keywords
+
+```bash
+GET https://api.ahrefs.com/v3/site-explorer/organic-keywords?target=example.com&mode=domain&country=us&limit=100
+
+Authorization: Bearer {api_token}
+```
+
+### Top pages
+
+```bash
+GET https://api.ahrefs.com/v3/site-explorer/top-pages?target=example.com&mode=domain&country=us&limit=50
+
+Authorization: Bearer {api_token}
+```
+
+### Keyword overview
+
+```bash
+GET https://api.ahrefs.com/v3/keywords-explorer/overview?keywords=keyword1,keyword2&country=us
+
+Authorization: Bearer {api_token}
+```
+
+### Keyword suggestions
+
+```bash
+GET https://api.ahrefs.com/v3/keywords-explorer/matching-terms?keyword=seed+keyword&country=us&limit=100
+
+Authorization: Bearer {api_token}
+```
+
+### SERP overview
+
+```bash
+GET https://api.ahrefs.com/v3/keywords-explorer/serp-overview?keyword=target+keyword&country=us
+
+Authorization: Bearer {api_token}
+```
+
+## Key Metrics
+
+### Domain Metrics
+- `domain_rating` - Domain Rating (DR)
+- `ahrefs_rank` - Ahrefs Rank
+- `referring_domains` - Referring domains count
+- `backlinks` - Total backlinks
+- `organic_traffic` - Estimated organic traffic
+
+### Keyword Metrics
+- `volume` - Monthly search volume
+- `keyword_difficulty` - KD score (0-100)
+- `cpc` - Cost per click
+- `clicks` - Estimated monthly clicks
+- `global_volume` - Global search volume
+
+### Backlink Fields
+- `url_from` - Source URL
+- `url_to` - Target URL
+- `anchor` - Anchor text
+- `domain_rating_source` - Source DR
+- `first_seen` - First discovery date
+
+## Modes
+
+- `domain` - Entire domain
+- `subdomains` - Domain + subdomains
+- `prefix` - URL prefix
+- `exact` - Exact URL
+
+## When to Use
+
+- Backlink analysis
+- Link building research
+- Keyword research
+- Competitive analysis
+- Content gap analysis
+- Site audits
+
+## Rate Limits
+
+- Varies by plan
+- 500-5000 rows per request
+
+## Relevant Skills
+
+- seo-audit
+- content-strategy
+- competitor-alternatives