bug: default agent has no session context

### Problem
if i create an agent with no memory, each invoke is a fresh session. For a new customer this may be confusing to know they actually need a memory for a basic agent.

To reproduce, create an agent without memory, run `agentcore invoke` or `agentcored dev` and observe the behavior.

Reproduce the issue with an agent without memory by running `agentcore invoke` and `agentcored dev`, then trace the TypeScript entry points for session handling. The issue does not specify the intended behavior or a test location, so confirm whether a default session context should persist before defining the regression test and completion criteria.

Written by the indexing model from the issue text.