Seeds nisse-core as the home for system-level design documentation. The
spec covers the shared library (Part A), both agents (Parts B and C), and
the supporting services, build order, and open decisions.
nisse-core holds this because the document describes the shared design
rather than either agent's implementation; the repo gains code later, at
the extraction step in the build order.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
