[Adapter] LangGraph export/runner

[shreyas-lyzr]

Summary

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

Details

LangGraph is a popular framework for building stateful, multi-agent applications with LLMs. A gitagent adapter would allow users to:

• Export: Convert a gitagent agent definition to a LangGraph graph/state machine
• Run: Execute a gitagent-defined agent using the LangGraph runtime

Acceptance Criteria

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

Resources

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