Fix Ruff format race condition

[emersonknapp]

Actually doing a few different housekeeping things here (sorry for overloaded, working in a hurry)

1. Fix a bug where sometimes .ruff.toml would be missing or partly written while checks running in parallel, leading to formatting going back and forth nondeterministically. Use os.rename for a tempfile which is an OS level atomic change, so there shouldn't be a partial or missing file at any point.
2. Add .envrc for direnv usage. Default exclude .envrc from shell and copyright checks - they aren't actually appropriate on it, this has come up in consuming repos.
3. Add Justfile and document dev workflows for using latest state on itself more easily. Move development to DEVELOPING.md, out of README.md
4. Bump patch version to 2.1.4
5. Add CI workflow that automatically creates and pushes tags when pyproject.toml version is bumped