docs: rewrite README + add DESIGN.md

README went from a pre-alpha stub to an action-oriented landing page:
what pk-auth is, a Mermaid diagram of the architecture, a
copy-paste demo command, the module layout, build prerequisites, and
pointers to the deeper docs.

DESIGN.md is new — a 14-section architecture overview aimed at a
developer adopting or extending pk-auth. Covers the three-ring
architecture (core / SPIs / adapters), the module map, the wire
contract every adapter mounts, the SPI catalog with required vs
optional flags, per-framework wiring snippets, persistence options,
the TS SDK, JWT model, security stance + pointers to the threat
model, build-system conventions, and code conventions
(records / sealed sums / JSpecify / Error Prone).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: wolpert