Restructure Repository into Installable Python Package

[bortiz-101]

Migrate to modern Python project structure:

• src/formallm/ layout
• pyproject.toml
• Editable install support

This formalizes FormaLLM as a Python library rather than a collection of scripts.

Acceptance Criteria

• pyproject.toml created
• pip install -e . works
• CLI entrypoint works (python -m formallm)
• Old script entrypoints removed or deprecated