Archive old agent directories after cost tracking refactor

[eltmon]

Problem

After work completes, agent directories in ~/.panopticon/agents/ accumulate indefinitely. Currently there are 200+ directories from historical work.

Blocked On

PAN-81: Event-sourced cost tracking

The current cost calculation (/api/costs/by-issue) iterates through agent directories to find workspace paths for session file parsing. Until PAN-81 implements hook-based cost collection with its own persistence, we cannot safely delete or archive agent directories without losing cost data.

Proposed Solution (After PAN-81)

1. Add status: "completed" to state.json when work finishes normally
2. Add "Archive old agents" button to Health tab UI
3. Implement archive logic:
   • Move directories older than X days (configurable) with status: "stopped" or "completed" to ~/.panopticon/agents-archive/
   • Or simply delete (cost data now lives in event log)
4. Optional: Auto-archive on dashboard startup

Related

• PAN-84 implemented filter-only approach as interim solution (hides old agents from Health tab)
• PAN-81 will eliminate dependency on agent directories for cost calculation

Acceptance Criteria

• Old agent directories can be cleaned up without losing cost data
• Archive button available in Health tab UI
• Configurable age threshold for archiving
• Confirmation dialog before archiving