build: switch hooks to prek, check 50/72

prek replaces pre-commit as the hook runner and reads the same
config. A new commit-msg hook in .prek/ rejects subjects over 50
characters, subjects without a module prefix, and body lines over
72 characters. Install the hooks with `uv run prek install`, and run
them with `just lint`.

pyproject.toml

@@ -40,7 +40,7 @@ requires = ["hatchling"]
 build-backend = "hatchling.build"
 
 [dependency-groups]
-dev = ["pre-commit>=2.16.0", "pytest>=8.0"]
+dev = ["prek>=0.2", "pytest>=8.0"]
 
 [tool.ruff]
 line-length = 79