feat(hosts): the Hosts list labels its figures and says how long; SLA tab gone; one place to add a host (#5279–#5283)

Hosts list:
- CPU and Memory are their own labelled columns (#5279); a silent
  agent's last readings are greyed, and Status says "agent silent for 7m".
- One uptime figure, 30 days, with 24h and 7d on hover. The SLA tab
  (/hosts/uptime) repeated it and is removed (#5280; one home per fact).
- Status is a word with how long ("Down for 12m"); host_status now carries
  down_since. Edit and Delete live on the host's page only (#5281).
- Figures are right-aligned with tabular digits (.table .num), not a
  monospace face.

Agent fleet (#5282): its add-host form is gone, so hosts are added only on
Hosts -> Add host. That form was also the only way to get a token for a
manual curl install, so that moves to the host's Agent card as "Install
manually" (POST /fleet/<host_id>/register), shown on the fleet page as
before. "Delete" there reads "Unregister".

Add host (#5283): laid out like the add-monitor form; a name clash comes
back inline with what was typed instead of a bare text 400; the address
is optional, as the host page already allowed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugins/host_agent/templates/settings_list.html

@@ -8,7 +8,8 @@
 </div>
 <p style="color:var(--text-muted);font-size:0.84rem;margin:-0.4rem 0 1.25rem;">
   Bulk agent operations across your inventory. For a single host, use its
-  <a href="/hosts/">host page</a> — provisioning and metrics live there.
+  <a href="/hosts/">host page</a>: provisioning, a manual install command and
+  metrics live there. New hosts are added with <a href="/hosts/new">Add host</a>.
 </p>
 
 {% if new_token %}
@@ -47,34 +48,19 @@
           </form>
           <form method="post" action="/plugins/host_agent/fleet/{{ item.reg.host_id }}/delete"
                 style="display:inline;"
-                data-confirm="Delete the agent registration for {{ item.name }}? The agent will be unable to report metrics until re-registered.">{{ csrf_field() }}
-            <button type="submit" class="btn btn-danger btn-sm">Delete</button>
+                data-confirm="Unregister the agent for {{ item.name }}? Its token stops working and metrics stop until it is registered again. The agent itself stays installed on the host.">{{ csrf_field() }}
+            <button type="submit" class="btn btn-danger btn-sm">Unregister</button>
           </form>
         </td>
       </tr>
       {% else %}
-      <tr><td colspan="7" class="empty">No hosts registered. Add one below.</td></tr>
+      <tr><td colspan="7" class="empty">No agents registered yet. <a href="/hosts/new">Add a host</a>, then install its agent from the host's page.</td></tr>
       {% endfor %}
     </tbody>
   </table>
   </div>
 </div>
 
-<div class="card">
-  <form method="post" action="/plugins/host_agent/fleet/add-host"
-        style="display:flex;gap:0.75rem;align-items:flex-end;flex-wrap:wrap;">{{ csrf_field() }}
-    <div class="form-group" style="margin-bottom:0;">
-      <label>Host name</label>
-      <input type="text" name="name" required>
-    </div>
-    <div class="form-group" style="margin-bottom:0;">
-      <label>Address (optional)</label>
-      <input type="text" name="address">
-    </div>
-    <button type="submit" class="btn">Add host</button>
-  </form>
-</div>
-
 {% if ansible_available %}
 {% set scope_select %}
         <label>Target</label>