Main session was unconditionally allowed through the boulder session guard,
causing continuation injection into sessions not part of the active boulder.
Now only sessions explicitly in boulder's session_ids (or background tasks)
receive boulder continuation, matching todo-continuation-enforcer behavior.
When boulderState.agent is not explicitly set (defaults to 'atlas'),
allow continuation for sessions where the last agent is 'sisyphus'.
This fixes the issue where boulder continuation was skipped when
Sisyphus took over the conversation after boulder creation.