[Adapter] LangChain export/runner

[shreyas-lyzr]

Summary

Add a LangChain adapter to gitagent, enabling export and run support for the LangChain framework.

Details

LangChain is one of the most widely adopted LLM frameworks. A gitagent adapter would allow users to:

• Export: Convert a gitagent agent definition to LangChain agent configuration
• Run: Execute a gitagent-defined agent using the LangChain runtime

Acceptance Criteria

• gitagent export --format langchain generates valid LangChain Python code
• gitagent run ./agent --adapter langchain executes the agent
• Documentation added to README adapter table
• Example added to examples/

Resources

• LangChain docs
• Existing adapters in src/adapters/ for reference