Rewrite README to reflect current stack features and add external links

Adds opskern.io, ebook (Leanpub), and status dashboard links. Restructures
content into What's Inside with full inventory of playbooks, roles, and
templates. Adds Features section covering self-healing, fleet monitoring,
update pipeline, TLS management, backups, and hardening. Links SECURITY.md,
CONTRIBUTING.md, CHANGELOG.md. Rewrites copy to match brand voice.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: thewismit