Rewrite README with ASCII diagrams and 5 scenario examples

Replaced Mermaid diagrams with ASCII-art flow diagrams for better
readability. Added 5 detailed scenarios: first-time login, API key
proxy, returning user, credential revocation, and audit trail.
Each scenario has a step-by-step flow diagram showing exactly what
each component sees (and doesn't see).

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

Author: kobzevvv