Use PyPI FreshForge optional dependency

[gparadis]

Summary

• add a PyPI-backed freshforge optional extra constrained to freshforge>=0.1.0a5,<0.2
• include FreshForge in dev and test extras so integration tests exercise the published alpha
• update FreshForge provider docs away from GitHub-direct install guidance
• keep core Modelwright dependencies and normal imports FreshForge-free

Closes #241.

Verification

• .venv/bin/python -m ruff check .
• .venv/bin/python -m pytest tests/test_freshforge_integration.py -q
• .venv/bin/python -m pytest
• .venv/bin/sphinx-build -q -b html docs _build/html -W
• .venv/bin/python scripts/verify_docs_theme.py _build/html
• scripts/check_release_artifacts.sh
• git diff --check