- Add `agents` field to tokens action=stats response (groups by agent
extracted from sessionId split on ':')
- Add new action=agent-costs returning per-agent stats, model breakdown,
session list, and daily cost/token timeline
- New AgentCostPanel with summary cards, pie chart, trend lines,
efficiency bars, and expandable ranking table
- Add nav-rail entry in OBSERVE group after Tokens
- Add ContentRouter case for agent-costs tab
- Add 5 E2E tests for the new API endpoints
Nyk