[FEATURE] Add system reminders

[ncrispino]

Feature Description

We need a principled way to add system reminders to massgen. Based on claude code, this is a feature where system reminder tags are added, e.g., for context awareness (MAS-77), human feedback (MAS-36), safety, memory reminders, etc.

We want a general framework to insert these mid-run while the LLM is streaming (like how we do with tool calls). Then, we can use these reminders however we want.