fix(supersession): one query for both directions, not two per note read

CI failed on 8d9e96c — eight tests in test_mcp_tool_notes.py, all
"Connect call failed (127.0.0.1, 5432)".

The proximate cause is that `_attach_supersession` runs on every note
read/write and those are unit tests of the tool layer with no database. But the
test failure exposed a worse decision underneath it.

I had written the two directions as two service calls, so every `get_note`
made TWO extra round trips plus TWO ACL checks — on the hottest path in the
product — to save a two-line partition in Python. That is the wrong trade
whether or not a test noticed.

`get_relations` replaces both: one OR query, one ACL check, partitioned by
which column holds the note's id. Its test asserts `execute.await_count == 1`,
so the collapse can't quietly come apart later.

The tests then get an autouse stub rather than the code getting a swallow. The
tool genuinely has a new dependency; hiding that behind a try/except to keep
unit tests green would be arranging for the code to lie about what it does.
This file already records the same hazard for note 2109, so the stub sits next
to that precedent.

Added the test that matters, which the first pass missed: a superseded record
still surfaces, so an agent WILL read stale material — and it must arrive with
a plain-language warning, not just a numeric field to notice. Also pinned that
both keys are ABSENT rather than present-and-empty when there are no relations.

Refs #278

tests/test_services_supersession.py

@@ -144,5 +144,32 @@ async def test_superseded_ids_is_empty_for_an_empty_candidate_set():
 async def test_reads_are_empty_when_the_caller_cannot_read_the_note():
     with patch("scribe.services.supersession.access.can_read_note",
                AsyncMock(return_value=False)):
-        assert await supersession.get_supersedes(7, 1) == []
-        assert await supersession.get_superseded_by(7, 1) == []
+        assert await supersession.get_relations(7, 1) == {
+            "supersedes": [], "superseded_by": []
+        }
+
+
+@pytest.mark.asyncio
+async def test_get_relations_partitions_both_directions_from_one_query():
+    """ONE round trip for both directions, because this runs on every note read.
+
+    Note 5 supersedes 2 and 3, and is itself superseded by 9. All four rows come
+    back from a single OR query and are partitioned by which column holds 5.
+    """
+    session = AsyncMock()
+    session.__aenter__ = AsyncMock(return_value=session)
+    session.__aexit__ = AsyncMock(return_value=False)
+    result = MagicMock()
+    result.all.return_value = [(5, 3), (5, 2), (9, 5)]   # (superseder, superseded)
+    session.execute = AsyncMock(return_value=result)
+
+    with patch("scribe.services.supersession.access.can_read_note",
+               AsyncMock(return_value=True)), \
+         patch("scribe.services.supersession.async_session", return_value=session):
+        rel = await supersession.get_relations(7, 5)
+
+    assert rel == {"supersedes": [2, 3], "superseded_by": [9]}
+    assert session.execute.await_count == 1, (
+        "both directions must come from one query — asking separately doubles "
+        "the round trips on the hottest path in the product"
+    )