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/settings_list.html

@@ -23,7 +23,7 @@
 {% endif %}
 
 <div class="card">
-  <form method="post" action="/plugins/host_agent/settings/add-host"
+  <form method="post" action="/plugins/host_agent/fleet/add-host"
         style="display:flex;gap:0.75rem;align-items:flex-end;flex-wrap:wrap;">
     <div class="form-group" style="margin-bottom:0;">
       <label>Host name</label>
@@ -155,12 +155,12 @@
         <td>{{ item.reg.distro or "—" }}</td>
         <td>{{ item.reg.last_seen_at.strftime("%Y-%m-%d %H:%M:%S") if item.reg.last_seen_at else "never" }}</td>
         <td class="td-actions">
-          <form method="post" action="/plugins/host_agent/settings/{{ item.reg.host_id }}/rotate-token"
+          <form method="post" action="/plugins/host_agent/fleet/{{ item.reg.host_id }}/rotate-token"
                 style="display:inline;"
                 onsubmit="return confirm('Rotate the token for this host? The existing agent will stop working until reinstalled with the new token.');">
             <button type="submit" class="btn btn-ghost btn-sm">Rotate token</button>
           </form>
-          <form method="post" action="/plugins/host_agent/settings/{{ item.reg.host_id }}/delete"
+          <form method="post" action="/plugins/host_agent/fleet/{{ item.reg.host_id }}/delete"
                 style="display:inline;"
                 onsubmit="return confirm('Delete this host registration? The agent will be unable to report metrics until re-registered.');">
             <button type="submit" class="btn btn-danger btn-sm">Delete</button>