test(cuda.core): add note about using multiple conftest modules

cuda_core/tests/AGENTS.md

@@ -65,3 +65,30 @@ by `cuCtxSynchronize()` before popping the context. Tests should not rely on
 that as a substitute for cleaning up explicitly: prefer context managers for
 resources whose lifetime fits a single scope, and keep pool lifetimes inside
 the test that creates them.
+
+## A per-directory `conftest.py` needs an `__init__.py`
+Under pytest's default `prepend` import mode, a test file's `sys.path` entry is
+the first parent directory *without* an `__init__.py`; the module is then named
+by its path relative to that directory. So for `tests/memory/test_x.py`:
+
+| | `sys.path` entry | module name | `import conftest` finds |
+|---|---|---|---|
+| no `__init__.py` | `tests/memory` | `test_x` | `tests/memory/conftest.py` |
+| with `__init__.py` | `tests` | `memory.test_x` | `tests/conftest.py` |
+
+Adding the `__init__.py` moves the insertion point up one level and solves two problems at once:
+
+1. Many modules do `from conftest import <helper>`, meaning the root
+   `tests/conftest.py`. Keeping the subdirectory off `sys.path` is what makes
+   that unambiguous. Otherwise a new per-directory `conftest.py` silently
+   captures the name and the import fails with `ImportError: cannot import name
+   ...` — reported in the *other* modules in that directory, not in the file you
+   added.
+2. Module names become hierarchical, so a basename reused under another
+   directory cannot collide. Flat names make `graph/test_memory.py` alongside
+   `tests/test_memory.py` an "import file mismatch" error.
+
+Split the two kinds of shared test code accordingly: fixtures and hooks go in
+`conftest.py`, which pytest discovers and nothing imports by name; constants and
+helper functions go in `tests/helpers/`, which tests import explicitly. Following
+that split also keeps the name collision from mattering in the first place.