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>

.claude-plugin/marketplace.json

@@ -6,13 +6,13 @@
   },
   "metadata": {
     "description": "Marketing skills for AI agents — conversion optimization, copywriting, SEO, paid ads, and growth",
-    "version": "2.0.1",
+    "version": "2.1.0",
     "repository": "https://github.com/coreyhaines31/marketingskills"
   },
   "plugins": [
     {
       "name": "marketing-skills",
-      "description": "40 marketing skills for technical marketers and founders: CRO, copywriting, cold email, SEO, AI SEO, paid ads, ad creative, video production, image generation, co-marketing, churn prevention, pricing, referrals, revenue operations, sales enablement, customer research, site architecture, and more",
+      "description": "41 marketing skills for technical marketers and founders: CRO, copywriting, cold email, SEO, AI SEO, paid ads, SMS, ad creative, video production, image generation, co-marketing, churn prevention, pricing, referrals, revenue operations, sales enablement, customer research, site architecture, and more",
       "source": "./"
     }
   ]