docs: LangChain memory replacement integration guide

[dgenio]

Context

Split from #80. A standalone integration guide for LangChain as a memory/context replacement.

Proposal

Create docs/integration_langchain.md with:

• Architecture diagram showing contextweaver replacing LangChain's ConversationBufferMemory / ConversationSummaryMemory
• Code example: LangChain agent using contextweaver for context compilation instead of built-in memory
• Key value proposition: budget-aware context vs LangChain's unbounded memory
• Migration path from existing LangChain memory to contextweaver

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

Acceptance Criteria

• docs/integration_langchain.md with architecture, code walkthrough, migration guide
• examples/langchain_demo.py demonstrating memory replacement pattern
• README examples table updated
• Example runs without errors (mock LLM calls)

File Paths

• docs/integration_langchain.md (new)
• examples/langchain_demo.py (new)
• README.md (edit)

Parent Issue

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