feat(hosts): Phase 1 — host detail hub page (unify host IA)

Make Hosts the front-and-center hub. A host now has a real detail page at
/hosts/<id> that pulls its facets into one view, instead of management being
scattered across a nav-less Host-Agents area and the edit form.

- hosts: new GET /hosts/<id> detail route + hosts/detail.html. Shows the
  monitors summary (ping/DNS status + latency + uptime 24h/7d/30d), an Ansible
  section (linked target, link/create, run-playbook), and an embedded Agent
  panel. Hosts list name links here; ansible-link redirects here.
- host_agent: GET /plugins/host_agent/panel/<host_id> — a self-contained HTMX
  fragment embedded into the core hub across the plugin boundary (core never
  imports plugin models). Shows live agent metrics + Update/Rotate/Remove when
  installed, or the provisioning path when not: inline "generate managed key"
  warning, a prompt to link an Ansible target first, then Provision (bootstrap
  password) / Install (managed key) tied to the host's target scope.

Part of milestone 70 (Hosts hub). Phase 2+ will enrich the list, redirect the
old fleet/settings pages, and re-taxonomize plugins into capabilities vs
integrations.

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

steward/templates/hosts/list.html

@@ -31,7 +31,7 @@
             {% set dns = latest_dns.get(host.id) %}
             {% set ut = uptime.get(host.id, {}) %}
             <tr>
-                <td>{{ host.name }}</td>
+                <td><a href="/hosts/{{ host.id }}" style="font-weight:600;">{{ host.name }}</a></td>
                 <td style="color:var(--text-muted);">{{ host.address }}</td>
                 <td style="color:var(--text-muted);font-size:0.85rem;">
                     {{ host.probe_type.value | upper }}{% if host.probe_type.value == 'tcp' %}:{{ host.probe_port }}{% endif %}