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`.

README.md

@@ -131,8 +131,8 @@ cd pq-cli
 # Install to a local venv:
 uv sync
 
-# Install pre-commit hooks:
-uv run pre-commit install
+# Install the git hooks (formatting, linting, commit messages):
+uv run prek install
 
 # Run the game and the tests:
 just run