docs: LangGraph state management integration guide

[dgenio]

Context

Split from #80. A standalone integration guide for LangGraph state management.

Proposal

Create docs/integration_langgraph.md with:

• Architecture diagram showing contextweaver integrated with LangGraph's state graph nodes
• Code example: LangGraph state graph using contextweaver for per-node context compilation
• Key patterns: routing in tool-selection nodes, firewall in tool-execution nodes, budget per graph branch
• Comparison: LangGraph's built-in state vs contextweaver's phase-aware compilation

Also create examples/langgraph_demo.py with a runnable example.

Acceptance Criteria

• docs/integration_langgraph.md with architecture, code walkthrough, comparison
• examples/langgraph_demo.py demonstrating state graph integration
• README examples table updated
• Example runs without errors (mock LLM calls)

File Paths

• docs/integration_langgraph.md (new)
• examples/langgraph_demo.py (new)
• README.md (edit)

Parent Issue

Part of #80. Tracked by #123 (framework integration guides epic).