feat(cli): rename to keccak256sum, add stdin and -c/--quiet/--status

- Rename keccak256 → keccak256sum (md5sum convention)
- Read hex from stdin when no positional args and stdin is piped
- Output follows md5sum format: <hash>  <label>
- -c <file>: verify checksums from file (two-space separated format)
- --quiet: suppress OK lines in check mode
- --status: suppress all output in check mode, exit code only
- Checksum files support comments (#) and multi-part hex inputs