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>

plugins/host_agent/templates/panel.html

@@ -37,11 +37,11 @@
       <button type="submit" class="btn btn-sm" title="Refresh agent.py + restart (token preserved)">Update agent</button>
     </form>
     {% endif %}
-    <form method="post" action="/plugins/host_agent/settings/{{ host.id }}/rotate-token" style="margin:0;"
+    <form method="post" action="/plugins/host_agent/fleet/{{ host.id }}/rotate-token" style="margin:0;"
           onsubmit="return confirm('Rotate token? The agent stops reporting until reinstalled/updated with the new token.');">
       <button type="submit" class="btn btn-sm btn-ghost">Rotate token</button>
     </form>
-    <form method="post" action="/plugins/host_agent/settings/{{ host.id }}/delete" style="margin:0;"
+    <form method="post" action="/plugins/host_agent/fleet/{{ host.id }}/delete" style="margin:0;"
           onsubmit="return confirm('Remove this agent registration? Metrics stop until re-registered.');">
       <button type="submit" class="btn btn-sm btn-danger">Remove</button>
     </form>
@@ -65,7 +65,7 @@
   {% elif not ansible_available %}
   <p style="color:var(--text-dim);font-size:0.85rem;">
     Ansible isn't available, so the agent can't be deployed from here. Install it manually with the
-    <a href="/plugins/host_agent/settings/">curl install command</a>.
+    <a href="/plugins/host_agent/fleet/">curl install command</a>.
   </p>
 
   {% elif not managed_key_set %}