Conda CI on GitHub Actions

Environment solving takes longer than the analysis itself.

Why it happens

The classic conda solver is slow and the package cache is not persisted.

What to change

1. Use mamba or the libmamba solver instead of the classic solver
2. Cache ~/conda_pkgs_dir keyed on the environment file
3. Pin the environment with an explicit lock file so solving is skipped entirely

Conda on runnerhut

jobs:
  test:
    runs-on: runnerhut-8vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
      - uses: conda-incubator/setup-miniconda@v3
        with: { miniforge-version: latest, use-mamba: true }
      - run: pytest -n auto

The only runnerhut-specific line is `runs-on: runnerhut-8vcpu-ubuntu-2404`. Everything else is standard GitHub Actions — the same actions, the same secrets and the same permissions model you use today.