test: check the domain layer against sys.stdlib_module_names

The layering test pinned an exact import set, so any new stdlib import in
skills/issue tripped it — 'collections', added by issue_ac.py, did. Assert
the rule AGENTS.md actually states (stdlib only, never subprocess) instead
of a frozen snapshot of it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/test_store_path.py

@@ -425,9 +425,11 @@ class TestLayering(unittest.TestCase):
                 for line in f:
                     if line.startswith(("import ", "from ")):
                         imported.add(line.split()[1].split(".")[0])
-        local = {"issue", "issue_index"}
-        self.assertEqual(imported - local, {"argparse", "os", "re", "sys"},
-                         "non-stdlib or unexpected import in the domain layer")
+        local = {"issue", "issue_ac", "issue_index"}
+        foreign = imported - local - sys.stdlib_module_names
+        self.assertEqual(foreign, set(),
+                         "non-stdlib import in the domain layer: %s"
+                         % ", ".join(sorted(foreign)))
         self.assertNotIn("subprocess", imported)