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/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.20.0",
+  "version": "0.21.0",
   "type": "module",
   "scripts": {
     "dev": "vite",