Use pytest-partition-check for CI patterns

[adamtheturtle]

What changed

• add pytest-partition-check==2026.8.10.1 to the development dependencies
• extract the CI pattern matrix from the GitHub Actions workflow with jq
• pipe those patterns into the pytest-check-partition CLI from a pre-push prek hook
• retain the repository-specific pytest-retry and pytest-beartype-tests exclusions and warning suppression
• remove the bespoke custom-linter pytest module and its dedicated CI shard

Why

pytest-partition-check packages the partition validation that originated in this repository. Calling its CLI directly from prek removes the repository-owned nested-pytest implementation and keeps the check close to the workflow configuration it validates.

Validation

• uv run --extra=dev prek run pytest-check-partition --all-files --hook-stage pre-push
• uv run --extra=dev prek run --all-files --hook-stage pre-commit
• uv run --extra=dev prek run --all-files --hook-stage pre-push