chore: modernize pre-commit hook configuration

The pre-commit config was broken on three fronts:

- `rev: main` / `rev: master` is rejected by modern pre-commit, which
  requires immutable tags (or a SHA) so hook versions are reproducible.
- `gitlab.com/pycqa/flake8` was archived years ago. flake8 now lives at
  `github.com/PyCQA/flake8`.
- `timothycrosley/isort` was renamed to `PyCQA/isort` and the old path
  no longer resolves reliably.

Pin every hook to its current latest release and replace `psf/black` with
`psf/black-pre-commit-mirror`, the upstream-recommended mirror that ships
without black's full dependency tree (faster hook installs).

- pre-commit/pre-commit-hooks → v6.0.0
- PyCQA/isort               → 8.0.1
- psf/black-pre-commit-mirror → 26.3.1
- PyCQA/flake8              → 7.3.0

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