Add JSON output to grut clean

[jongio]

Add a --json flag to grut clean so setup scripts can inspect reclaimable transient data before deleting it. The command should report each target, whether it exists, its byte count, and whether it was removed.

Acceptance criteria:

• grut clean --json previews without deleting.
• grut clean --force --json deletes the same targets as text mode.
• JSON includes total bytes, target count, and per-target status.