Add top-N limits to dispatch stats

[jongio]

Problem

dispatch stats can print long repository, branch, and host breakdowns for users with a large session history. That makes the command noisy in a status pane, README snippet, or quick daily check.

Proposed solution

Add --top <n> to cap each breakdown list to the first N entries after the existing count sort. Apply the same cap to text and JSON output. Leave totals and the activity calendar unchanged.

Acceptance criteria

• dispatch stats --top 5 prints at most five rows in each breakdown section.
• dispatch stats --json --top 5 caps by_repository, by_branch, and by_host_type arrays.
• --top 0 or a negative value returns a clear error.
• Calendar output still covers the full filtered session set.
• Tests cover parsing, text output, JSON output, and invalid values.

Complexity

S

Priority

Medium