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>

AGENTS.md

@@ -152,6 +152,34 @@ Follow the [Conventional Commits](https://www.conventionalcommits.org/) specific
 - [ ] `SKILL.md` is under 500 lines
 - [ ] No sensitive data or credentials
 
+## Tool Integrations
+
+This repository includes a tools registry for agent-compatible marketing tools.
+
+- **Tool discovery**: Read `tools/REGISTRY.md` to see available tools and their capabilities
+- **Integration details**: See `tools/integrations/{tool}.md` for API endpoints, auth, and common operations
+- **MCP-enabled tools**: ga4, stripe, mailchimp, google-ads, resend, zapier
+
+### Registry Structure
+
+```
+tools/
+├── REGISTRY.md              # Index of all tools with capabilities
+└── integrations/            # Detailed integration guides
+    ├── ga4.md
+    ├── stripe.md
+    ├── rewardful.md
+    └── ...
+```
+
+### When to Use Tools
+
+Skills reference relevant tools for implementation. For example:
+- `referral-program` skill → rewardful, tolt, dub-co, mention-me guides
+- `analytics-tracking` skill → ga4, mixpanel, segment guides
+- `email-sequence` skill → customer-io, mailchimp, resend guides
+- `paid-ads` skill → google-ads, meta-ads, linkedin-ads guides
+
 ## Skill Categories
 
 See `README.md` for the current list of skills organized by category. When adding new skills, follow the naming patterns of existing skills in that category.