Add LangChain-based agent example

[ponderingdemocritus]

Summary

Provide a LangChain-powered example that demonstrates how to build and expose an agent using the Lucid Agents framework.

Requirements

• Create a minimal, clean example (for example in examples/langchain/).
• Showcase a single HTTP route that exposes the agent.
• Ensure the example runs end-to-end with documented setup steps (environment variables, installs, commands).
• Add concise README or inline comments explaining how the LangChain integration works within Lucid Agents.

Acceptance Criteria

• Running the documented steps spins up a working agent server backed by LangChain.
• Route responds successfully and demonstrates meaningful agent behavior.
• Example passes linting/tests (if applicable) and fits the repository's style guidelines.