docs: add a runnable LangChain integration example

[nqzai]

Add examples/langchain_example.py showing verify_agent_scope guarding a LangChain tool. Keep it minimal and runnable.

Acceptance

• examples/langchain_example.py present + referenced in README
• Uses the real async client API

Good first issue.

[abubakrrrx]

I'd like to work on this issue.
I have experience with LangChain and have already
prepared a runnable example using KakuninToolGuard
with async client API. Will submit a PR shortly.

[nqzai]

You are most welcome