Add grouped event summary endpoint for agent triage

[spinchange]

Problem

Repeated warnings and errors currently need to be grouped by the agent, which is noisy and token-expensive.

Proposal

Add a new get_umi_event_summary endpoint that rolls up events by source and event ID.

Acceptance Criteria

• New get_umi_event_summary endpoint exists.
• Supports lookback window, level filters, grouping, and top-N.
• Returns grouped counts with first seen, last seen, and representative sample message.
• Repeated event floods are summarized rather than dumped raw by default.