Dashboard: per-host disk usage (0.21.0)

The per-host resource bar gained a Disk stat (used / total). The agent now
reports disk_total/disk_used via shutil.disk_usage on its stacks dir (a host
bind-mount), alongside the existing cpu/mem/containers fields; the local host
uses the existing system info disk data.

- agent_app.py: _disk_info() + disk_total/disk_used in _system_info().
- Frontend: ResourceBar gained diskUsed/diskTotal (5-column grid); AgentSystem
  type gained disk_total/disk_used.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/api/agents.ts

@@ -11,6 +11,8 @@ export interface AgentSystem {
   cpu_cores: number;
   mem_total: number;
   mem_used: number;
+  disk_total: number;
+  disk_used: number;
   containers_running: number;
   containers_total: number;
 }