Track specialist costs with time period filtering

[eltmon]

Summary

Add the ability to view specialist agent costs over different time periods, similar to how we track issue agent costs.

Requirements

API Enhancements

• Extend /api/specialists/:name/cost to accept time period filters:

  • ?period=today - costs from today
  • ?period=week - costs from this week
  • ?period=month - costs from this month
  • ?period=all - all-time costs (default, current behavior)

• Add /api/specialists/costs/summary endpoint:

  • Returns aggregated costs for all specialists
  • Supports same time period filters
  • Shows breakdown by specialist

Dashboard UI

• Add cost summary section to specialists panel
• Time period selector (Today / This Week / This Month / All Time)
• Cost breakdown by specialist with visual chart/bars
• Total specialist cost prominently displayed

Data Model

Consider whether to:

1. Parse JSONL files on-demand (current approach, slower for large files)
2. Maintain a cost tracking database/file that gets updated incrementally

Technical Notes

• Specialist JSONL files contain costUSD field per entry
• Need to filter entries by timestamp
• May need to cache/index for performance with large session files

Acceptance Criteria

• Can view specialist costs filtered by time period
• Dashboard shows cost breakdown by specialist
• Cost data refreshes automatically (polling)
• Performance acceptable even with large session files