Add API & Webhooks guide

Comprehensive guide covering the WebSocket control plane (chat, skill_invoke,
heartbeat_trigger, status), webhook ingress/egress configuration, Node.js and
Python client libraries, integration patterns (GitHub Actions, Slack, Home
Assistant, n8n, PagerDuty, Datadog), custom HTTP channel adapters, MCP server
exposure, authentication, reverse proxy setup, rate limiting, remote access
(SSH tunnel, Tailscale, Cloudflare), error handling, and testing.

Closes #43

Author: melvincarvalho