`cache-dir / config-dir` in `pyproject.toml` so tools stop littering the project root

Problem Every dev tool invents its own cache/state directory and drops it straight in the project root: .pytest_cache/, .mypy_cache/, .ruff_cache/, .tox/, .hypothesis/, htmlcov/, .coverage. Each one needs its own .gitignore line and its own cleanup logic in CI. Some tools already let you relocate their own cache (mypy: cache_dir, ruff: cache-dir, pytest: cache_dir ini option) — but it’s N separate settings for N tools, with no shared default. Proposal Two well-known keys, directly under [tool],...