feat(session_search): add recent sessions mode when query is omitted

When session_search is called without a query (or with an empty query),
it now returns metadata for the most recent sessions instead of erroring.
This lets the agent quickly see what was worked on recently without
needing specific keywords.

Returns for each session: session_id, title, source, started_at,
last_active, message_count, preview (first user message).
Zero LLM cost — pure DB query. Current session lineage and child
delegation sessions are excluded.

The agent can then keyword-search specific sessions if it needs
deeper context from any of them.

tools/session_search_tool.py

@@ -179,6 +179,58 @@ async def _summarize_session(
                 return None
 
 
+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)  # fetch extra to skip current
+
+        # Resolve current session lineage to exclude it
+        current_root = None
+        if current_session_id:
+            try:
+                sid = current_session_id
+                visited = set()
+                while sid and sid not in visited:
+                    visited.add(sid)
+                    s = db.get_session(sid)
+                    parent = s.get("parent_session_id") if s else None
+                    sid = parent if parent else None
+                current_root = max(visited, key=len) if visited else current_session_id
+            except Exception:
+                current_root = current_session_id
+
+        results = []
+        for s in sessions:
+            sid = s.get("id", "")
+            if current_root and (sid == current_root or sid == current_session_id):
+                continue
+            # Skip child/delegation sessions (they have parent_session_id)
+            if s.get("parent_session_id"):
+                continue
+            results.append({
+                "session_id": sid,
+                "title": s.get("title") or None,
+                "source": s.get("source", ""),
+                "started_at": s.get("started_at", ""),
+                "last_active": s.get("last_active", ""),
+                "message_count": s.get("message_count", 0),
+                "preview": s.get("preview", ""),
+            })
+            if len(results) >= limit:
+                break
+
+        return json.dumps({
+            "success": True,
+            "mode": "recent",
+            "results": results,
+            "count": len(results),
+            "message": f"Showing {len(results)} most recent sessions. Use a keyword query to search specific topics.",
+        }, ensure_ascii=False)
+    except Exception as e:
+        logging.error("Error listing recent sessions: %s", e, exc_info=True)
+        return json.dumps({"success": False, "error": f"Failed to list recent sessions: {e}"}, ensure_ascii=False)
+
+
 def session_search(
     query: str,
     role_filter: str = None,
@@ -195,11 +247,14 @@ def session_search(
     if db is None:
         return json.dumps({"success": False, "error": "Session database not available."}, ensure_ascii=False)
 
+    limit = min(limit, 5)  # Cap at 5 sessions to avoid excessive LLM calls
+
+    # Recent sessions mode: when query is empty, return metadata for recent sessions.
+    # No LLM calls — just DB queries for titles, previews, timestamps.
     if not query or not query.strip():
-        return json.dumps({"success": False, "error": "Query cannot be empty."}, ensure_ascii=False)
+        return _list_recent_sessions(db, limit, current_session_id)
 
     query = query.strip()
-    limit = min(limit, 5)  # Cap at 5 sessions to avoid excessive LLM calls
 
     try:
         # Parse role filter
@@ -385,7 +440,7 @@ SESSION_SEARCH_SCHEMA = {
         "properties": {
             "query": {
                 "type": "string",
-                "description": "Search query — keywords, phrases, or boolean expressions to find in past sessions.",
+                "description": "Search query — keywords, phrases, or boolean expressions to find in past sessions. Omit to list recent sessions instead (no search, just metadata).",
             },
             "role_filter": {
                 "type": "string",
@@ -397,7 +452,7 @@ SESSION_SEARCH_SCHEMA = {
                 "default": 3,
             },
         },
-        "required": ["query"],
+        "required": [],
     },
 }
 
@@ -410,7 +465,7 @@ registry.register(
     toolset="session_search",
     schema=SESSION_SEARCH_SCHEMA,
     handler=lambda args, **kw: session_search(
-        query=args.get("query", ""),
+        query=args.get("query") or "",
         role_filter=args.get("role_filter"),
         limit=args.get("limit", 3),
         db=kw.get("db"),