Document test organization and add Cursor workflow rules.

Record the one-to-one unit test file convention in README and add rules for test placement and copy-ready PR text after first push.

Co-authored-by: Cursor <[email protected]>

.cursor/rules/test-organization.mdc

@@ -0,0 +1,31 @@
+---
+description: Unit test file naming and placement conventions
+globs: tests/**/*.py
+alwaysApply: false
+---
+
+# Test Organization
+
+Unit tests use a one-to-one mapping between source modules and test files.
+
+## Naming
+
+- `python_repositories/<path>/<module>.py` → `tests/unit/<module>_test.py`
+- Test files must end with `_test.py` (enforced by pre-commit `name-tests-test`)
+
+## Placement
+
+- Add tests to the existing `*_test.py` for the module under test
+- Do not create cross-cutting test files (e.g. `structural_typing_test.py`); colocate with the relevant interface/adapter/config test file
+- Shared fixtures → `tests/conftest.py`
+- Module-specific helper classes → the matching test file
+
+## Examples
+
+```
+json_repository_interface.py  →  tests/unit/json_repository_interface_test.py
+redis_adapter.py              →  tests/unit/redis_adapter_test.py
+redis_config.py               →  tests/unit/redis_config_test.py
+```
+
+Integration tests live under `tests/integration/<backend>/` with the same `_test.py` suffix.