feat(dashboard): unified Hosts widget + grouped widget picker

Bring the dashboard in line with the unified host IA + improved displays.

- New core "Hosts — Overview" widget (/hosts/overview/widget): one row per host
  combining monitor status (ping dot + latency, uptime 24h) with the agent
  glance (CPU / memory / disk root + stale flag), each row linking to the host
  hub. Reads agent data from the generic PluginMetric table via a core-safe
  _agent_overview_by_host helper (no host_agent import); freshness vs the
  plugin's stale window. The granular Ping/DNS/Uptime/Agent widgets stay.
- Group the add-widget picker into Core monitors / Monitoring capabilities /
  Integrations (a `group` field on every WIDGET_REGISTRY entry + section
  headings in _edit_panels.html), matching the Settings → Plugins taxonomy.
- Fix the agent fleet widget rows to link to the host hub (/hosts/<id>) instead
  of the old /plugins/host_agent/<id>/ page.

Scribe #903.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

steward/core/widgets.py

@@ -32,6 +32,16 @@ WIDGET_REGISTRY: dict[str, dict] = {
         "poll": True,
         "params": [],
     },
+    "hosts_overview": {
+        "key": "hosts_overview",
+        "label": "Hosts — Overview",
+        "description": "One row per host: monitor status + uptime and agent CPU/memory/disk, linking to the host hub",
+        "hx_url": "/hosts/overview/widget",
+        "detail_url": "/hosts/",
+        "plugin": None,
+        "poll": True,
+        "params": [],
+    },
     "uptime_summary": {
         "key": "uptime_summary",
         "label": "Uptime / SLA",
@@ -292,8 +302,34 @@ WIDGET_REGISTRY: dict[str, dict] = {
 }
 
 
+# Group each widget for the picker (Core monitors / Monitoring capabilities /
+# Integrations), mirroring the Settings → Plugins taxonomy. Capability plugins
+# are host/monitoring facets; traefik/unifi are discrete vendor integrations.
+_CORE_WIDGETS = {"ping", "dns", "status_overview", "uptime_summary", "hosts_overview"}
+_INTEGRATION_PLUGINS = {"traefik", "unifi"}
+GROUP_LABELS = {
+    "core": "Core monitors",
+    "capability": "Monitoring capabilities",
+    "integration": "Integrations",
+}
+GROUP_ORDER = ["core", "capability", "integration"]
+
+
+def _group_for(w: dict) -> str:
+    if w["key"] in _CORE_WIDGETS:
+        return "core"
+    if w.get("plugin") in _INTEGRATION_PLUGINS:
+        return "integration"
+    return "capability"
+
+
+for _w in WIDGET_REGISTRY.values():
+    _w["group"] = _group_for(_w)
+
+
 def register_widget(definition: dict) -> None:
     """Register a widget at runtime (used by external plugins)."""
+    definition.setdefault("group", _group_for(definition))
     WIDGET_REGISTRY[definition["key"]] = definition