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>

README.md

@@ -37,6 +37,7 @@ Skills reference each other and build on shared context. The `product-marketing`
 │schema    │ │paywalls  │ │social    │ │            │ │community │ │competitors  │ │           │
 │content   │ │          │ │video     │ │            │ │lead-magnt│ │comp-profile │ │           │
 │aso       │ │          │ │image     │ │            │ │co-mktg   │ │directory    │ │           │
+│          │ │          │ │sms       │ │            │ │          │ │             │ │           │
 └────┬─────┘ └────┬─────┘ └────┬─────┘ └─────┬──────┘ └────┬─────┘ └──────┬──────┘ └─────┬─────┘
      │            │            │              │             │              │              │
      └────────────┴─────┬──────┴──────────────┴─────────────┴──────────────┴──────────────┘
@@ -93,6 +94,7 @@ See each skill's **Related Skills** section for the full dependency map.
 | [seo-audit](skills/seo-audit/) | When the user wants to audit, review, or diagnose SEO issues on their site. Also use when the user mentions "SEO... |
 | [signup](skills/signup/) | When the user wants to optimize signup, registration, account creation, or trial activation flows. Also use when the... |
 | [site-architecture](skills/site-architecture/) | When the user wants to plan, map, or restructure their website's page hierarchy, navigation, URL structure, or internal... |
+| [sms](skills/sms/) | When the user wants to plan, build, or optimize SMS or MMS marketing — including welcome flows, abandoned cart texts,... |
 | [social](skills/social/) | When the user wants help creating, scheduling, or optimizing social media content for LinkedIn, Twitter/X, Instagram,... |
 | [video](skills/video/) | When the user wants to create, generate, or produce video content using AI tools or programmatic frameworks. Also use... |
 <!-- SKILLS:END -->