Create a virtual environment and run a tool in an isolated env with uv venv and uv run

Documented steps

1. Create a standalone environment with uv venv, which defaults to .venv.
2. Run commands inside the project environment with uv run <command>, or activate it first.
3. Run a one-off dependency-injected tool with uvx <tool>, for example uvx ruff check, which runs in a temporary environment.
4. For pip-style control, use uv pip install, uv pip list, and uv pip freeze against the current environment.