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/integrations/postscript.md

@@ -0,0 +1,126 @@
+# Postscript
+
+SMS marketing platform built for Shopify direct-to-consumer brands. Deepest Shopify integration of any SMS platform.
+
+## Capabilities
+
+| Integration | Available | Notes |
+|-------------|-----------|-------|
+| API | ✓ | REST API |
+| MCP | - | Not available |
+| CLI | - | None |
+| SDK | - | Use API directly |
+
+## Authentication
+
+- **Type**: API Key
+- **Header**: `Authorization: Bearer {api_key}` or `X-Postscript-Api-Key: {api_key}`
+- **Get key**: Postscript dashboard → Settings → API
+- **Note**: Keys are scoped per shop
+
+## Common Agent Operations
+
+### Search subscribers
+
+```bash
+GET https://api.postscript.io/api/v2/subscribers?phone_number=%2B15551234567
+```
+
+### Create subscriber (opt-in)
+
+```bash
+POST https://api.postscript.io/api/v2/subscribers
+
+{
+  "phone_number": "+15551234567",
+  "email": "user@example.com",
+  "first_name": "Jane",
+  "subscribed_at": "2026-05-15T10:00:00Z",
+  "opt_in_source": "checkout_keyword"
+}
+```
+
+Must include valid opt-in metadata for TCPA compliance.
+
+### Unsubscribe
+
+```bash
+DELETE https://api.postscript.io/api/v2/subscribers/{subscriberId}/subscription
+```
+
+### List keywords (e.g., JOIN, SAVE)
+
+```bash
+GET https://api.postscript.io/api/v2/keywords
+```
+
+### Send transactional message
+
+```bash
+POST https://api.postscript.io/api/v2/transactional/sms
+
+{
+  "phone_number": "+15551234567",
+  "message": "Your order #1234 shipped. Track at https://..."
+}
+```
+
+Transactional requires separate enablement; counts under transactional consent.
+
+### List campaigns
+
+```bash
+GET https://api.postscript.io/api/v2/campaigns
+```
+
+### List automations (flows)
+
+```bash
+GET https://api.postscript.io/api/v2/automations
+```
+
+### Webhooks
+
+Subscribe to events: `subscriber.created`, `subscriber.unsubscribed`, `message.delivered`, `message.failed`, `conversion.attributed`.
+
+## API Pattern
+
+REST + JSON. Standard `Bearer` auth. Pagination via `cursor` and `limit` (max 100).
+
+## Key Features
+
+- Native Shopify integration: purchases, abandoned carts, browse, product catalog auto-sync
+- Strong abandoned cart and browse abandonment automation builders
+- AI Reply (auto-reply trained on brand voice)
+- Conversational SMS / live agent for two-way
+- Opt-in tools: popups, keyword opt-in, checkout opt-in
+- A2P 10DLC managed in-platform
+- Reporting: revenue, click-through, conversion attribution, opt-out rate
+
+## Pricing
+
+- Plans: Starter (free, 1K msgs/mo), Growth ($100+/mo), Professional, Enterprise
+- Per-send pricing on top: ~$0.015 SMS, ~$0.04 MMS
+- Annual contracts standard at Growth+
+- Pricing scales meaningfully past 50K subscribers
+
+## When to Use
+
+- Shopify DTC brand wanting SMS-specific tooling (vs combined email/SMS)
+- Need deep abandoned cart, browse abandonment, post-purchase automation out of the box
+- Want managed A2P 10DLC + compliance tools
+- Mid-size DTC brand (10K–500K SMS subscribers)
+
+## When NOT to Use
+
+- Non-Shopify ecom — integration is shallow
+- Already on Klaviyo for email and SMS is secondary — Klaviyo SMS is simpler
+- Mid-market/enterprise needing concierge support — Attentive
+- Custom platform or B2B SaaS — Twilio
+
+## Relevant Skills
+
+- sms
+- emails (run alongside via Klaviyo or similar)
+- churn-prevention (win-back flows)
+- onboarding (post-purchase activation)