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",