Move scikit-learn to core dependencies

[chicham]

Context

HallucinationDetector(BaseEstimator, ClassifierMixin) and EntropyFeatureExtractor(BaseEstimator, TransformerMixin) are introduced in #239. Once these classes exist, every import artefactual requires sklearn. Currently sklearn is optional under [calibration].

This issue must be merged before #239 — otherwise the first commit importing BaseEstimator will break installs without the [calibration] extra.

Changes

• Move scikit-learn>=1.6.0 from [project.optional-dependencies.calibration] to [project.dependencies] in pyproject.toml
• Remove sklearn from [calibration] optional deps (it is now always present)
• Verify check_estimator is runnable without installing the [calibration] extra

Acceptance criteria

• pip install artefactual (no extras) installs sklearn
• from artefactual import EPR works with no extras installed
• CI matrix runs check_estimator in the base install