fix(state): add missing thread locks to 4 SessionDB methods

search_sessions(), clear_messages(), delete_session(), and
prune_sessions() all accessed self._conn without acquiring self._lock.
Every other method in the class uses the lock. In multi-threaded
contexts (gateway serving concurrent platform messages), these
unprotected methods can cause sqlite3.ProgrammingError from concurrent
cursor operations on the same connection.

hermes_state.py

@@ -809,6 +809,7 @@ class SessionDB:
         offset: int = 0,
     ) -> List[Dict[str, Any]]:
         """List sessions, optionally filtered by source."""
+        with self._lock:
             if source:
                 cursor = self._conn.execute(
                     "SELECT * FROM sessions WHERE source = ? ORDER BY started_at DESC LIMIT ? OFFSET ?",
@@ -871,6 +872,7 @@ class SessionDB:
 
     def clear_messages(self, session_id: str) -> None:
         """Delete all messages for a session and reset its counters."""
+        with self._lock:
             self._conn.execute(
                 "DELETE FROM messages WHERE session_id = ?", (session_id,)
             )
@@ -882,6 +884,7 @@ class SessionDB:
 
     def delete_session(self, session_id: str) -> bool:
         """Delete a session and all its messages. Returns True if found."""
+        with self._lock:
             cursor = self._conn.execute(
                 "SELECT COUNT(*) FROM sessions WHERE id = ?", (session_id,)
             )
@@ -900,6 +903,7 @@ class SessionDB:
         import time as _time
         cutoff = _time.time() - (older_than_days * 86400)
 
+        with self._lock:
             if source:
                 cursor = self._conn.execute(
                     """SELECT id FROM sessions