Add a limit flag to crash report listing

[jongio]

Crash report history can grow over time, and scripts often need only the newest entries.
Add a limit flag to grut report --list that trims both text and JSON list output.

Acceptance criteria:

• grut report --list --limit N prints at most N reports.
• grut report --list --json --limit N encodes at most N reports.
• --limit 0 keeps the current unlimited behavior.
• Negative limits return a clear error.