title: Write a small function and its tests
check: python -m pytest -q .bench_check
plan: |
  Milestone 1: add a slugify function.

  1. Create `slugify.py` in the workspace root, defining `slugify(text: str) -> str`.
  2. It lowercases the text, replaces every run of characters that are not a-z or
     0-9 with a single hyphen, and strips hyphens from both ends. Examples:
     "Hello, World!" -> "hello-world"; "  Nisse  Agent  " -> "nisse-agent";
     "a--b" -> "a-b"; "" -> "".
  3. Create `test_slugify.py` with pytest tests covering the four examples.
  4. Run `python -m pytest -q test_slugify.py`, and fix anything until it passes.
  5. Call finish_milestone, with final yes.