refactor(hosts): remove legacy host_agent redirects/paths (no back-compat)

Dev-only instance, no bookmarks — per family rule 22, fully remove old paths
instead of shimming them.

- Delete the /plugins/host_agent/ (index) and /plugins/host_agent/settings/
  redirect routes; delete the now-dead host_list.html fleet template.
- Move the remaining management POST routes off /settings/ to /fleet/
  (add-host, rotate-token, delete) — single canonical prefix.
- Repoint real callers to canonical URLs: dashboard widgets (host resources →
  /hosts/, history → /plugins/host_agent/fleet/), the full-metrics page
  breadcrumb + back link (→ the host hub), Settings→Ansible link, and the
  agent panel's curl-install link.

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

steward/core/widgets.py

@@ -262,7 +262,7 @@ WIDGET_REGISTRY: dict[str, dict] = {
         "label": "Host Agent — Resources",
         "description": "Fleet view: CPU, memory, disk, and load per monitored host",
         "hx_url": "/plugins/host_agent/widget",
-        "detail_url": "/plugins/host_agent/",
+        "detail_url": "/hosts/",
         "plugin": "host_agent",
         "poll": True,
         "params": [],
@@ -272,7 +272,7 @@ WIDGET_REGISTRY: dict[str, dict] = {
         "label": "Host Agent — History",
         "description": "CPU/memory/disk history for one host",
         "hx_url": "/plugins/host_agent/widget/history",
-        "detail_url": "/plugins/host_agent/settings/",
+        "detail_url": "/plugins/host_agent/fleet/",
         "plugin": "host_agent",
         "poll": True,
         "params": [