List crash reports as JSON

[jongio]

Crash report automation needs a stable list format instead of parsing the human table. Add grut report --list --json so scripts can enumerate stored crash reports before deciding which one to show or file.

Acceptance criteria:

• grut report --list --json prints a JSON array of crash report summaries.
• The JSON output is empty array when no reports exist.
• Existing human output for grut report --list is unchanged.
• Tests cover the flag and JSON formatting.