Langgraph Implementation

This implements LangGraph's stateful agent graphs with proper checkpointing and human-in-the-loop patterns. You get concrete guidance on state schemas (TypedDict vs Pydantic), conditional routing, and the Command/Send patterns for dynamic flows. The implementation gates are the standout here: four sequenced checks that force you to verify thread IDs are stable, state contains what your next node expects, and you've actually inspected pending work before resuming. It distinguishes between InMemorySaver for tests and durable checkpointers for production, which matters when you're debugging why state disappeared. Covers streaming modes, subgraphs, and the practical difference between interrupt() calls and interrupt_before/after configuration.