feat: add host_agent plugin — push-model host resource monitoring

Lightweight stdlib-only Python agent runs on each monitored host,
collects CPU / memory / disk / load / uptime from /proc every 30s,
and POSTs signed payloads to the Roundtable ingest endpoint. One-line
curl-pipe installer creates a hardened systemd unit; admin UI manages
host registrations with rotate / revoke.

- agent.py: 370 LoC single-file daemon, ring buffer + exponential backoff
- ingest route: bearer-token auth, metric expansion into plugin_metrics
- install.sh.j2: systemd unit with NoNewPrivileges / ProtectSystem / ProtectHome
- settings UI: add host / rotate token / delete registration (admin-only)
- dashboard widgets: fleet-glance table + per-host history chart
- stale-agent scheduler: 60s log warning for agents past 180s silence

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

host_agent/plugin.yaml

@@ -0,0 +1,19 @@
+# plugins/host_agent/plugin.yaml
+name: host_agent
+version: "1.0.0"
+description: "Remote Linux host resource monitoring via a lightweight Python push agent (CPU, memory, storage, load, uptime)"
+author: "Roundtable"
+license: "MIT"
+min_app_version: "0.1.0"
+repository_url: "https://git.fabledsword.com/bvandeusen/Roundtable-plugins"
+homepage: "https://git.fabledsword.com/bvandeusen/Roundtable-plugins/src/branch/main/host_agent"
+tags:
+  - host
+  - monitoring
+  - cpu
+  - memory
+  - storage
+
+config:
+  stale_after_seconds: 180
+  default_interval_seconds: 30