Add README

Names the artifact and points at the specification in docs/.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

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+# nisse-core
+
+The shared library both Nisse agents sit on (Part A of the specification), and the
+home of the system-level design documentation.
+
+The library itself does not exist yet: it is extracted from `nisse-dev` once that
+agent works, at step 10 of the build order. Until then this repository carries the
+specification.
+
+## Architecture
+
+    docs/Architecture-Build-Spec.md
+
+Covers the shared library (Part A), the Dev Agent `nisse-dev` (Part B), the Sysadmin
+Agent `nisse-ops` (Part C), the supporting services, the build order, and the
+remaining open decisions.