Update README.md to Clarify Mutmut Mutation Testing (#36)

Author: rxdt

README.md

@@ -197,7 +197,7 @@ Note: The worker must be installed and authenticated separately. **Claude Code e
 - Edit rules at [pyproject.toml](pyproject.toml) for [ruff](https://docs.astral.sh/ruff/), [pylint](https://pypi.org/project/pylint/), [pydoclint](https://pypi.org/project/pydoclint/0.9.1/), [pyright](https://github.com/microsoft/pyright), [pytest](https://docs.pytest.org/en/stable/), [hypothesis](https://hypothesis.readthedocs.io/), [complexipy](https://github.com/rohaquinlop/complexipy), [mutmut](https://mutmut.readthedocs.io/)
 - Add forbidden files, directories, or patterns in `[tool.harness.gate]` at [pyproject.toml](pyproject.toml)
 - Add [Hypothesis](https://hypothesis.readthedocs.io/) tests in any test directory, examples at [test_properties.py](tests/preferences/test_properties.py).
-- Run [mutmut](https://mutmut.readthedocs.io/) by hand with `uv run mutmut run`, then `uv run mutmut browse`. A surviving mutant is a covered line no assertion checks. It is not a gate check: `mutmut run` exits 0 even with survivors. Its verdicts are cached against source hashes and ignore test edits, so re-run a single mutant by name once you have written a test for it. Examples at [test_with_mutations.py](tests/preferences/test_with_mutations.py).
+- Run [mutmut](https://mutmut.readthedocs.io/) by hand with `uv run mutmut run`, then `uv run mutmut browse`. A surviving mutant is a covered line no assertion checks. It is not a gate check: `mutmut run` exits 0 even with survivors and it should be run ~1x/week. Its mutants are cached in a JSON and should be used to identify weak tests. Example at [check_mutmut.py](https://github.com/rxdt/loopgate_harness/blob/main/mutation/check_mutmut.py).
 - [semgrep](https://docs.semgrep.dev/semgrep-ci/sample-ci-configs) has no repo config here. It uses registry configs / Semgrep's built-in defaults which ignore tests.
 - Update `[tool.harness.gate.checks]` in [pyproject.toml](pyproject.toml). [ci.yml](.github/workflows/ci.yml) runs those **same exact** `harness gate` checks.
 - Add or remove coding preferences [preferences.py](preferences/preferences.py) that only agents in loops **must** respect. Current preferences: