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/templates/settings/ansible.html

@@ -29,7 +29,7 @@
       Steward's reusable Ansible identity. Provisioning installs the <strong>public</strong>
       key into each host's <code>{{ ssh_user }}</code> account; steady-state runs authenticate
       with the private half (stored encrypted). Generate it once, then provision hosts from
-      <a href="/plugins/host_agent/settings/">Host Agents</a>.
+      <a href="/plugins/host_agent/fleet/">Host Agents</a>.
     </p>
     {% if ssh_public_key %}
     <div class="form-group">