docs: add runnable LangChain integration example

[abubakrrrx]

Closes #2

Added examples/langchain_example.py showing:

• Agent registration with Kakunin async client
• KakuninToolGuard wrapping two LangChain tools
• Scope enforcement before every tool call
• Behavioral event recording with risk_band
• Full async client API usage with ChatGroq LLM

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[abubakrrrx]

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