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>

backend/agent_app.py

@@ -12,6 +12,7 @@ from __future__ import annotations
 
 import logging
 import os
+import shutil
 from dataclasses import asdict
 
 import tempfile
@@ -61,7 +62,7 @@ def _map_docker(exc: DockerError):
         raise HTTPException(status_code=code, detail=exc.detail or exc.error)
     raise exc  # falls through to the global 502 DockerError handler
 
-AGENT_VERSION = "0.20.0"
+AGENT_VERSION = "0.21.0"
 
 
 # --------------------------------------------------------------------------- #
@@ -180,6 +181,17 @@ def _mem_info() -> tuple[int, int]:
     return 0, 0
 
 
+def _disk_info() -> tuple[int, int]:
+    """Return (total_bytes, used_bytes) for the host disk backing the stacks dir."""
+    for path in (settings.STACKS_DIR, "/"):
+        try:
+            usage = shutil.disk_usage(path)
+            return usage.total, usage.used
+        except OSError:
+            continue
+    return 0, 0
+
+
 def _system_info() -> dict:
     docker_version = ""
     host_os = ""
@@ -194,6 +206,7 @@ def _system_info() -> dict:
     except DockerError as exc:
         docker_version = f"unavailable ({exc.error})"
     mem_total, mem_used = _mem_info()
+    disk_total, disk_used = _disk_info()
     return {
         "hostname": _hostname(),
         "docker_version": docker_version,
@@ -201,6 +214,8 @@ def _system_info() -> dict:
         "cpu_cores": os.cpu_count() or 0,
         "mem_total": mem_total,
         "mem_used": mem_used,
+        "disk_total": disk_total,
+        "disk_used": disk_used,
         "containers_running": running,
         "containers_total": total,
     }

backend/main.py

@@ -55,7 +55,7 @@ async def lifespan(app: FastAPI):
     schedule_task.cancel()
 
 
-app = FastAPI(title="StackPilot", version="0.20.0", lifespan=lifespan)
+app = FastAPI(title="StackPilot", version="0.21.0", lifespan=lifespan)
 
 app.add_middleware(
     CORSMiddleware,