feat(settings): Phase 4 — capabilities vs integrations + nav cleanup

Final phase of the host-IA unification (milestone 70).

- Settings → Plugins split into two tiers: "Monitoring capabilities"
  (host_agent, http, snmp, docker — built-in host facets, surfaced via
  Hosts/Status, on by default) and "Integrations" (traefik, unifi — external
  systems, off until configured). Presentation only: a CAPABILITY_PLUGINS set
  (overridable by plugin.yaml `kind:`) tags each plugin; module loading,
  optional deps, and migrations are untouched.
- Drop the "default-enable a plugin" framing in the UI copy — capabilities are
  described as built-in, not optional add-ons.
- Nav: remove the standalone "Uptime" item (folded into Hosts; still reachable
  via the SLA button on the Hosts list).

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

steward/templates/base.html

@@ -209,7 +209,6 @@ textarea { resize: vertical; }
     <a href="/">Dashboard</a>
     <a href="/status">Status</a>
     <a href="/hosts/">Hosts</a>
-    <a href="/hosts/uptime">Uptime</a>
     <a href="/ping/">Ping</a>
     <a href="/dns/">DNS</a>
     <a href="/alerts/">Alerts</a>