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/host_detail.html

@@ -1,7 +1,7 @@
 {% extends "base.html" %}
 {% from "_macros.html" import crumbs %}
 {% block title %}{{ host.name }} — Host Agent — Steward{% endblock %}
-{% block breadcrumb %}{{ crumbs([("Host Agents", "/plugins/host_agent/"), (host.name, "")]) }}{% endblock %}
+{% block breadcrumb %}{{ crumbs([("Hosts", "/hosts/"), (host.name, "/hosts/" ~ host.id), ("Metrics", "")]) }}{% endblock %}
 
 {% macro fmt_bps(v) %}
 {%- if v is none -%}—
@@ -27,7 +27,7 @@
 {% block content %}
 <div style="display:flex;align-items:center;justify-content:space-between;gap:1rem;flex-wrap:wrap;margin-bottom:0.75rem;">
   <div style="display:flex;align-items:center;gap:0.6rem;">
-    <a href="/plugins/host_agent/" class="btn btn-ghost btn-sm">← Fleet</a>
+    <a href="/hosts/{{ host.id }}" class="btn btn-ghost btn-sm">← Host</a>
     <h1 class="page-title" style="margin-bottom:0;">{{ host.name }}</h1>
     {% if stale %}<span class="badge badge-red">stale</span>{% else %}<span class="badge badge-green">live</span>{% endif %}
   </div>