Add a UI

[simonmesmith]

• Remove run.py from root
• Move run.py functionality to agentflow/cli.py
• Import cli.main into agentflow/__init__.py so users can run the CLI with python -m agentflow
• Create a new ui folder in the root
• Build a Flask- or FastAPI-based app here that users can run with python -m ui.run
• Update requirements as needed, e.g. with FastAPI and Uvicorn
• Move root/tests to root/agentflow/tests for agentflow-specific tests (update file references accordingly, including in README)

[simonmesmith]

I probably need to create a project for this and then group issues into that project that are related, since there are a lot of things I need to do here.