Run `mypy` in CI?

Add to `.pre-commit-config.yaml` to enforce that:
```
- repo: https://github.com/pre-commit/mirrors-mypy
rev: v1.13.0
hooks:
- id: mypy
files: "src"
```

Open .pre-commit-config.yaml and review the existing hooks before adding the proposed mypy hook for files under src. Run the pre-commit checks and confirm that mypy is invoked in CI without configuration errors.

Written by the indexing model from the issue text.