fix(memory): archive short idle sessions for Dream

tests/agent/test_autocompact_unit.py

@@ -405,13 +405,37 @@ class TestCheckExpired:
         scheduler.assert_not_called()
         assert "dream:20260602-155256" not in ac._archiving
 
-    def test_already_trimmed_session_skips(self):
-        """Expired session with no removable tail should not be re-scheduled."""
+    def test_short_unarchived_session_schedules(self):
+        """A short idle session still needs an archive entry for Dream."""
+        ac = _make_autocompact(ttl=15)
+        mock_sm = MagicMock(spec=SessionManager)
+        last_active = datetime(2026, 1, 1, 10, 0, 0)
+        session = _make_session("cli:short", updated_at=last_active)
+        _add_turns(session, 2)
+        mock_sm.list_sessions.return_value = [
+            {"key": "cli:short", "updated_at": last_active.isoformat()},
+        ]
+        mock_sm.get_or_create.return_value = session
+        ac.sessions = mock_sm
+
+        scheduled = []
+
+        def scheduler(coro):
+            scheduled.append(coro)
+            coro.close()
+
+        ac.check_expired(scheduler, _runtime)
+
+        assert len(scheduled) == 1
+        assert ac._archiving == {"cli:short"}
+
+    def test_fully_archived_session_skips(self):
         ac = _make_autocompact(ttl=15)
         mock_sm = MagicMock(spec=SessionManager)
         last_active = datetime(2026, 1, 1, 10, 0, 0)
         session = _make_session("cli:done", updated_at=last_active)
         _add_turns(session, 2)
+        session.last_consolidated = len(session.messages)
         mock_sm.list_sessions.return_value = [
             {"key": "cli:done", "updated_at": last_active.isoformat()},
         ]