Add check mode to grut clean

[jongio]

grut clean can preview transient data, but scripts cannot currently fail when cleanup is needed.
Add a check mode that exits non-zero when sessions or diagnostics are present without deleting anything.

Acceptance criteria:

• grut clean --check previews the same targets as the default command.
• It exits successfully when there is nothing to clean.
• It returns an error when cleanup targets are present.
• It works with --json.