Start user-engine implementation scaffold

docs/development.md

@@ -0,0 +1,42 @@
+# Development
+
+## Stack
+
+The initial implementation uses Python 3.12 and the standard library. The
+first slice intentionally avoids runtime dependencies so the repository can be
+tested immediately in local and agent environments.
+
+## Layout
+
+```text
+src/user_engine/
+  domain/      transport- and persistence-neutral domain schemas
+  ports.py     adapter protocols for identity, authorization, events, audit,
+               membership export, application bindings, and secrets
+  testing/     local fixtures for tests and examples
+tests/         standard-library unittest suite
+```
+
+The domain layer should not import HTTP frameworks, database clients, or
+platform-specific SDKs. Those integrations belong behind ports.
+
+## Commands
+
+```bash
+make test
+```
+
+The command runs:
+
+```bash
+PYTHONPATH=src python3 -m unittest discover -s tests -p 'test_*.py'
+```
+
+## Implementation Rule
+
+Add new behavior in this order:
+
+1. domain schema or port;
+2. local fixture or adapter;
+3. test that proves the boundary;
+4. infrastructure adapter or API surface.