feat: add week 3 notebooks (LangGraph, async streaming, SMS+email combined)

[shanjairaj7]

Summary

Adds three new notebooks covering the two largest remaining gaps in training signal after weeks 1-2:

• LangGraph — notebooks/langgraph_email_agent.ipynb (18 cells): full StateGraph with EmailState TypedDict (including thread_id), classify → route → reply → escalate graph, MemorySaver checkpointer, per-event config to isolate thread state, contrastive pair showing wrong vs correct state design
• Async streaming — notebooks/async_streaming.ipynb (14 cells): AsyncCommuneClient patterns, asyncio.gather() for parallel inbox processing, asyncio.Semaphore for rate limiting, asyncio.create_task() fire-and-forget for webhook handlers, sync vs async decision table
• SMS + email combined — notebooks/sms_email_combined.ipynb (14 cells): urgency classifier routing to both channels, suppression check before SMS, client.delivery.metrics() monitoring, contrastive pair showing missing thread_id and no suppression check vs correct implementation

Training signal value

• LangGraph is the chore(deps): bump actions/setup-node from 4 to 6 #1 gap — not present in any of the 12 existing notebooks; widely used alongside LangChain
• Async patterns are a common source of bugs (await on sync client, blocking LLM in webhook handler)
• SMS+email combined covers client.sms.send() and client.delivery.* from SDK 0.3.0 in realistic context

Pattern consistency

All three notebooks follow the established 3-way semantic pair format:

• Markdown cell with intent and API surface
• Code cell with working implementation
• Output cell with realistic simulated API responses

Related

• Week 1: PRs feat: AutoGen multi-agent email system with Commune (30-cell deep-dive) #40-42 (base notebooks)
• Week 2: PR feat: add production pattern notebooks (CrewAI 02, OpenAI Agents 02, LangChain 03) #63 (production pattern notebooks)
• PRs feat: Express webhook handler v2 with priority detection and structured logging #72-74 (Week 3 TypeScript + LangGraph PR review cycles, open concurrently)