Add short output to grut status

[jongio]

grut status has readable text and JSON, but prompt scripts and quick terminal checks need compact dirty-file lines.
Add a short mode for concise status output.

Acceptance criteria:

• grut status --short prints branch information and compact file lines.
• Clean repositories print a single clean line.
• --check still returns an error when the report is dirty.
• --short cannot be combined with --json.