fix(agent): bound per-session file state

nanobot/command/builtin.py

@@ -302,6 +302,7 @@ async def cmd_new(ctx: CommandContext) -> OutboundMessage:
     """Stop active task and start a fresh session."""
     loop = ctx.loop
     await loop._cancel_active_tasks(ctx.key)  # pyright: ignore[reportPrivateUsage]
+    loop.discard_session_file_state(ctx.key)
     session = ctx.session or loop.sessions.get_or_create(ctx.key)
     snapshot = session.messages[session.last_consolidated:]
     runtime = None