chore(git): add commit message normalizer

[marcus]

Summary

• add a local commit-msg hook that normalizes safe conventional-commit cases and rejects malformed subjects
• install the new hook via make, add regression coverage, and document the repo commit message policy
• update GoReleaser changelog filters so scoped docs/test/ci/chore commits stay excluded

Testing

• make test-commit-msg
• go test ./...
• manual git commit --allow-empty hook checks for rejection and auto-normalization