Simplify the pre-commit hooks

- run "format" and "lint" separately
  => remove the nox session "pre-commit"
- execute the entire "test-suite" before merges
  + rename "pre-merge" into "test-suite"
  + do not "format" and "lint" here any more
  + do not execute this before pushes
    * allow branches with <100% test coverage to exist on GitHub
      (they cannot be merged into 'main' until 100% coverage)
    * GitHub Actions executes the test suite