Add LangChain integration example

[base76-research-lab]

Context

LangChain is the most widely used LLM framework. Many users will want to plug NoUse into an existing LangChain chain or agent.

Task

Create `examples/langchain_example.py` showing how to use NoUse as a context injector in a LangChain pipeline.

Suggested approach

```python
from langchain_openai import ChatOpenAI
from langchain_core.messages import SystemMessage, HumanMessage
import nouse

brain = nouse.attach()
llm = ChatOpenAI(model="gpt-4.1-mini")

question = "Explain the relationship between attention and memory in transformers."
context = brain.query(question).context_block()

messages = [
SystemMessage(content=context),
HumanMessage(content=question),
]

response = llm.invoke(messages)
print(response.content)
```

Notes

• Keep it minimal — the pattern is the point, not the chain depth
• A comment explaining where NoUse fits (context injector before the LLM call) is more valuable than boilerplate