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

tools/session_search_tool.py

@@ -266,7 +266,11 @@ _HIDDEN_SESSION_SOURCES = ("tool",)
 def _list_recent_sessions(db, limit: int, current_session_id: str = None) -> str:
     """Return metadata for the most recent sessions (no LLM calls)."""
     try:
-        sessions = db.list_sessions_rich(limit=limit + 5, exclude_sources=list(_HIDDEN_SESSION_SOURCES))  # fetch extra to skip current
+        sessions = db.list_sessions_rich(
+            limit=limit + 5,
+            exclude_sources=list(_HIDDEN_SESSION_SOURCES),
+            order_by_last_active=True,
+        )  # fetch extra to skip current
 
         # Resolve current session lineage to exclude it
         current_root = None