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`.

.pre-commit-config.yaml

@@ -1,3 +1,6 @@
+default_install_hook_types: [pre-commit, commit-msg]
+default_stages: [pre-commit]
+
 repos:
 
 - repo: https://github.com/astral-sh/ruff-pre-commit
@@ -6,3 +9,11 @@ repos:
   - id: ruff-check
     args: [--fix]
   - id: ruff-format
+
+- repo: local
+  hooks:
+  - id: commit-msg-50-72
+    name: Commit message 50/72 rule
+    entry: python3 .prek/check_commit_msg
+    language: system
+    stages: [commit-msg]