feat: add commit-msg hook, improve robustness

- Add commit-msg hook enforcing Conventional Commits format
  (allows merge, fixup, and squash commits)
- Fix file size display: use MiB (binary) to match the binary limit calculation
- Make gitleaks optional: skip gracefully if not installed
- Suppress stderr on mg register in pre-push
- Add shellcheck directive to pre-commit
- Mark gitleaks as optional in README prerequisites
- Clean up FUNDING.yml boilerplate

pre-push

@@ -1,5 +1,5 @@
 #!/bin/sh
 
-mg register >/dev/null &
+mg register >/dev/null 2>&1 &
 test "$(git lfs track | wc -l)" -gt 0 || exit 0
 git lfs pre-push "$@"