Commit message normalizer

[marcus]

Summary

• add a local commit-msg hook and install it alongside the existing pre-commit hook
• document the normalized commit subject format and the allowed exempt flows
• reject malformed trailing refs like (random), (#abc), and (td-) while keeping valid td and PR refs working

Testing

• make install-hooks
• make test-commit-msg
• go test ./...
• manual smoke test for valid and invalid commit subjects from a linked worktree