Cookbook — Graph workflow patterns reference

[AmitoVrito]

Create docs/cookbook/graph-patterns.md:

15+ ready-to-copy patterns:

1. Linear 3-node graph
2. Conditional routing
3. Parallel fan-out with merge
4. Looping graph with max_steps
5. Graph with SQLite checkpoint
6. Graph with Redis checkpoint
7. Human-in-the-loop with approval_node
8. Subgraph composition
9. Graph with token streaming
10. Graph with SSE streaming (FastAPI)
11. Graph with WebSocket streaming
12. Graph with execution tracing
13. Graph with event callbacks
14. TypedState with custom reducers
15. Graph exported as Mermaid diagram

[AmitoVrito]

Completed in PR #96 — docs/cookbook/graph-patterns.md has 15 graph workflow patterns.