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

.github/FUNDING.yml

@@ -1,12 +1 @@
-# These are supported funding model platforms
-
-github: taigrr # Replace with up to 4 GitHub Sponsors-enabled usernames e.g., [user1, user2]
-patreon: # Replace with a single Patreon username
-open_collective: # Replace with a single Open Collective username
-ko_fi: # Replace with a single Ko-fi username
-tidelift: # Replace with a single Tidelift platform-name/package-name e.g., npm/babel
-community_bridge: # Replace with a single Community Bridge project-name e.g., cloud-foundry
-liberapay: # Replace with a single Liberapay username
-issuehunt: # Replace with a single IssueHunt username
-otechie: # Replace with a single Otechie username
-custom: # Replace with up to 4 custom sponsorship URLs e.g., ['link1', 'link2']
+github: taigrr