perf: lazy session creation — defer DB write until first message

Remove eager create_session() call from AIAgent.__init__(). Sessions
are now created lazily on first _flush_messages_to_session_db() call
via ensure_session() which uses INSERT OR IGNORE.

Impact: eliminates ~32% of sessions that are created at agent init
but never receive any messages.

run_agent.py

@@ -1093,30 +1093,10 @@ class AIAgent:
         self._session_db = session_db
         self._parent_session_id = parent_session_id
         self._last_flushed_db_idx = 0  # tracks DB-write cursor to prevent duplicate writes
-        if self._session_db:
-            try:
-                self._session_db.create_session(
-                    session_id=self.session_id,
-                    source=self.platform or os.environ.get("HERMES_SESSION_SOURCE", "cli"),
-                    model=self.model,
-                    model_config={
-                        "max_iterations": self.max_iterations,
-                        "reasoning_config": reasoning_config,
-                        "max_tokens": max_tokens,
-                    },
-                    user_id=None,
-                    parent_session_id=self._parent_session_id,
-                )
-            except Exception as e:
-                # Transient SQLite lock contention (e.g. CLI and gateway writing
-                # concurrently) must NOT permanently disable session_search for
-                # this agent.  Keep _session_db alive — subsequent message
-                # flushes and session_search calls will still work once the
-                # lock clears.  The session row may be missing from the index
-                # for this run, but that is recoverable (flushes upsert rows).
-                logger.warning(
-                    "Session DB create_session failed (session_search still available): %s", e
-                )
+        # Lazy session creation: defer until first message flush.
+        # _flush_messages_to_session_db() calls ensure_session() which uses
+        # INSERT OR IGNORE — creating the row only when messages arrive.
+        # This eliminates ~32% of sessions that are created but never used.
         
         # In-memory todo list for task planning (one per agent/session)
         from tools.todo_tool import TodoStore