feat: add sms skill + 5 SMS platform integration docs (#307)

New skill: skills/sms/
- SKILL.md (336 lines, under 500 limit): strategy, when SMS beats email,
  compliance overview, sequence types, copy guidelines, platform selection,
  measurement, common mistakes
- references/compliance.md: TCPA, A2P 10DLC, EU GDPR, CASL, Australia,
  opt-in disclosure templates, STOP/HELP response templates, audit checklist
- references/sequence-templates.md: full copy templates with character
  counts for welcome, abandoned cart, browse abandonment, post-purchase,
  win-back, promotional, transactional, re-engagement, replenishment, VIP
- references/platforms.md: platform deep-dives across Klaviyo SMS,
  Postscript, Attentive, Twilio, Plivo, AudienceTap, Brevo, SimpleTexting,
  Customer.io
- evals/evals.json: 6 evals covering getting started, abandoned cart,
  TCPA compliance, opt-out diagnosis, A2P throughput, emoji segment cost

New integration docs:
- tools/integrations/twilio.md
- tools/integrations/postscript.md
- tools/integrations/attentive.md
- tools/integrations/plivo.md
- tools/integrations/audiencetap.md

Registry + marketplace wiring:
- tools/REGISTRY.md: 5 new entries + new SMS / Messaging category section
- .claude-plugin/marketplace.json: bumped to 2.1.0, SMS added to plugin description
- VERSIONS.md: sms 1.0.0 + 2.1.0 changelog entry
- README.md: skill table re-sorted via sync-skills.js, sms added to ASCII flow

All 41 skills pass validation. sync-skills.js is idempotent on current state.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

tools/REGISTRY.md

@@ -54,6 +54,11 @@ Quick reference for AI agents to discover tool capabilities and integration meth
 | postmark | Email | ✓ | - | [✓](clis/postmark.js) | ✓ | [postmark.md](integrations/postmark.md) |
 | brevo | Email/SMS | ✓ | - | [✓](clis/brevo.js) | ✓ | [brevo.md](integrations/brevo.md) |
 | activecampaign | Email/CRM | ✓ | - | [✓](clis/activecampaign.js) | ✓ | [activecampaign.md](integrations/activecampaign.md) |
+| twilio | SMS/Voice | ✓ | - | ✓ | ✓ | [twilio.md](integrations/twilio.md) |
+| plivo | SMS/Voice | ✓ | - | - | ✓ | [plivo.md](integrations/plivo.md) |
+| postscript | SMS | ✓ | - | - | - | [postscript.md](integrations/postscript.md) |
+| attentive | SMS | ✓ | - | - | - | [attentive.md](integrations/attentive.md) |
+| audiencetap | SMS/Email | ✓ | - | - | - | [audiencetap.md](integrations/audiencetap.md) |
 | hunter | Email Outreach | ✓ | - | [✓](clis/hunter.js) | - | [hunter.md](integrations/hunter.md) |
 | snov | Email Outreach | ✓ | - | [✓](clis/snov.js) | - | [snov.md](integrations/snov.md) |
 | lemlist | Email Outreach | ✓ | - | [✓](clis/lemlist.js) | - | [lemlist.md](integrations/lemlist.md) |
@@ -189,6 +194,23 @@ Email marketing, transactional email, and automation platforms.
 
 **Agent recommendation**: Resend for transactional (dev-friendly). Postmark for deliverability. Customer.io for advanced automation. Kit for creators. Beehiiv for newsletters. Klaviyo for e-commerce email/SMS. ActiveCampaign for email + CRM combo.
 
+### SMS / Messaging
+
+SMS and MMS marketing platforms and programmable messaging APIs.
+
+| Tool | Best For | MCP Available |
+|------|----------|:-------------:|
+| **klaviyo** | DTC ecom already on Klaviyo email | - |
+| **postscript** | Shopify DTC, SMS-first depth | - |
+| **attentive** | Mid-market+ DTC, full-service | - |
+| **twilio** | Custom API builds, transactional, dev-first | - |
+| **plivo** | Twilio alternative, lower per-send cost | - |
+| **audiencetap** | DTC with AI-forward creative + on-pack QR opt-in | - |
+| **brevo** | EU SMB email + SMS combo | - |
+| **customer-io** | Behavior-based SMS automation | - |
+
+**Agent recommendation**: Klaviyo SMS for ecom already on Klaviyo email. Postscript for Shopify-first depth. Attentive for mid-market+ wanting concierge support. Twilio (or Plivo for lower cost) for custom builds and transactional/auth. AudienceTap when AI creative or on-pack QR opt-in matters.
+
 ### Advertising
 
 Paid advertising platforms and campaign management.