feat: redesign Plugin Metrics page as Host Overview

Replace pill-tab plugin view with an accordion layout that shows key
metrics (CPU%, MEM%, top disk%, net delta, nagios status) at a glance
in each host card header. Plugin sections expand as structured tables.

- Rename page to "Host Overview" (URL /plugins unchanged)
- Three-wave parallel data loading: glance plugins on host expand,
  on-demand fetch for filesystem_info and extras
- Per-plugin table renderers with inline percent bars and threshold
  colour coding
- Add escHtml() for XSS-safe rendering of all field values
- Remove stale planning docs (REFACTORING.md, hbd/Plan.md)

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

hbd/server/http.py

@@ -520,8 +520,8 @@ async def start(
 
         tmpl = env.get_template("plugins.html")
         body = tmpl.render(
-            title="Plugin Metrics - Heartbeat",
-            header="Plugin Metrics",
+            title="Host Overview - Heartbeat",
+            header="Host Overview",
             hosts=hosts_with_plugins,
             current_user=current_user.to_dict() if current_user else None,
             active_page="plugins",

hbd/server/templates/nav.html

@@ -4,7 +4,7 @@
   </button>
   <div class="nav-links" id="nav-links">
     <a href="/live"{% if active_page == "live" %} class="active"{% endif %}>Live Dashboard</a>
-    <a href="/plugins"{% if active_page == "plugins" %} class="active"{% endif %}>Plugin Metrics</a>
+    <a href="/plugins"{% if active_page == "plugins" %} class="active"{% endif %}>Host Overview</a>
     <a href="/alerts"{% if active_page == "alerts" %} class="active"{% endif %}>Alerts</a>
     {% if current_user and current_user.admin %}
     <a href="/settings"{% if active_page == "settings" %} class="active"{% endif %}>Settings</a>