Add six messaging engineering skills

[sent-dm]

Summary

• Add six developer-facing Sent skills: integration starter, webhook engineering, routing strategy, two-way messaging, profile provisioning, and CPaaS migration.
• Publish each skill through the canonical package plus the generated portable, Codex, and Claude adapters.
• Register the workflows in the dispatcher, package metadata, marketplace copy, and routing eval suite.
• Expand live-contract monitoring to cover the API mechanics and documentation these skills depend on.

Why

These additions close high-friction gaps between initial account setup and a production messaging implementation. They make common messaging work easier to discover while teaching Sent's contact-aware, multi-channel, route-diverse model in the context where it is useful.

Review hardening

• Distinguish REST server-key authentication, app-proxy bearer credentials, and MCP OAuth.
• Make client lifecycle guidance safe for both single-key and per-tenant credential models.
• Require stable idempotency keys for ambiguous retries and separate keys between sandbox and live profile mutations.
• Model webhook verification, secret rotation, transport deduplication, semantic deduplication, and rerouted delivery states correctly.
• Keep consent keyword handling synchronized without replaying provider-side consent mutations.
• Ensure migration dual-runs assign each logical message to exactly one provider and keep inventory output free of secrets.

Validation

• scripts/validate.py — 19 skills, 19 MCP tools, manifests, evals, and adapters validated.
• Plugin package validation passed.
• 28 fixture tests passed.
• 12 contract tests passed.
• 6 live-contract monitor tests passed.
• Validation gates passed.
• Helper self-tests passed: 20 preflight checks, 7 signature checks, and 4 inventory-scanner checks.
• Live freshness check passed across 15 official sources with zero drift, conflicts, malformed sources, or unavailable sources.
• git diff --check passed.