test: stub the auto-inject supersession lookup where there is no database

CI on f20c019: nine failures across three files, all on the auto-inject path.
Step 3's own tests passed; these are the same shape as the step 2 breakage —
a real database call added to a path whose unit tests run without one.

Stubbed per file, each saying why, rather than once in conftest. A global stub
would hide the dependency from every future test on these paths too, which is
the same "make the code lie" trade refused in 984407f, one level up.

The auto-inject query is kept separate from the ranker's rather than threaded
through, and that is deliberate: `_reserve_slot_for_reuse` runs a SECOND search
and can add a hit to the menu, so the final `kept` set is not a subset of what
the ranker scored. Labelling whatever actually reached the menu needs its own
lookup over that final set — one indexed query on a handful of ids.

Refs #278

tests/test_note_usage.py

@@ -9,6 +9,22 @@ from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
 
+
+@pytest.fixture(autouse=True)
+def _no_supersession():
+    """The auto-inject menu now asks which of its lines are superseded (#278).
+
+    That is a real database call on a path these tests exercise without one.
+    Stubbed to "nothing superseded" — the ordinary state — rather than hidden
+    behind a try/except in the product, which would make the code lie about
+    what it does. The label's own behaviour is covered in
+    tests/test_supersession_ranking.py.
+    """
+    with patch("scribe.services.plugin_context.superseded_ids",
+               AsyncMock(return_value=set())):
+        yield
+
+
 from scribe.services import note_usage
 from scribe.services.note_usage import (
     empty_usage,