fix(session_search): order recent mode by last activity instead of start time

- order session_search recent-mode results by last activity instead of session start time
- add an opt-in `order_by_last_active` path to `SessionDB.list_sessions_rich`
- add regression coverage for both the database ordering and recent-mode call path

tests/tools/test_session_search.py

@@ -242,6 +242,21 @@ class TestSessionSearchConcurrency:
 
 
 class TestRecentSessionListing:
+    def test_recent_mode_requests_last_active_ordering(self):
+        from unittest.mock import MagicMock
+
+        mock_db = MagicMock()
+        mock_db.list_sessions_rich.return_value = []
+
+        result = json.loads(_list_recent_sessions(mock_db, limit=5))
+
+        assert result["success"] is True
+        mock_db.list_sessions_rich.assert_called_once_with(
+            limit=10,
+            exclude_sources=["tool"],
+            order_by_last_active=True,
+        )
+
     def test_current_child_session_excludes_root_lineage_even_when_child_id_is_longer(self):
         from unittest.mock import MagicMock