Normalize commit message format

[marcus]

Summary

• add a local commit-msg hook that normalizes safe subject fixes and rejects malformed spaced scopes or ref-only summaries
• install the new hook via make install-hooks, add regression coverage, and keep changelog filtering aligned with scoped commit types
• document the repo commit subject policy in the README and loop prompt

Testing

• make test-commit-msg
• go test ./...
• go build ./...
• make install-hooks
• manual git commit --allow-empty smoke checks for malformed scope rejection, ref-only rejection, and safe normalization