Add langgraph agentic files to repo

Add 3 levels of langgraph agents:

- Simple agentic loop on mathematical expressions (to understand how agents work, beginner friendly)
- Simple v0 agent that does basic operations agentically on timeseries data (we build up from here)
- Attempt at recreating [InsightPilot](https://arxiv.org/pdf/2304.00477) (buggy, needs to be improved, we can optionally build from here too)