feat(monitor): active push loop replacing standalone monitor

Adds a periodic POST loop to <backend>/monitor/server/heartbeat so
HF plugin can take over the standalone harborforge-monitor daemon's
job — same X-API-Key header, same flat telemetry shape (cpu_pct /
mem_pct / disk_pct / swap_pct / load_avg / uptime_seconds /
plugin_version / agents[]). HF backend stays unchanged.

Config: monitor_push_enabled (default false; opt-in to avoid surprise
heartbeats from existing deployments), monitor_push_interval_seconds
(default 30), reuses apiKey for the X-API-Key header. Lift the
container's HF_MONITER_API_KEY into config.apiKey, flip
monitor_push_enabled true, then docker rm -f the container — DB
last_seen_at keeps advancing under the plugin's loop.

Collector grew swap + cpu sampling (two reads of /proc/stat over a
1-second window when SampleCPU=true). Bridge endpoint stays cheap
(SampleCPU=false on demand); push loop is the only caller paying the
sampling cost.

E2E in sim: monitor_push_enabled=true + apiKey from injected
MonitoredServer row → server_states.last_seen_at advances exactly
every interval_seconds (10s configured, 10s observed). cpu/mem/disk/
swap_pct all populate correctly.

README.md

@@ -18,9 +18,19 @@ submodule of the HarborForge umbrella repo.
 
 ## What it does
 
-- **Monitor bridge** — HTTP server on `127.0.0.1:<monitor_port>` that
-  responds to `/telemetry` with a Snapshot the HarborForge.Monitor
-  binary expects (system metrics + every Plexum agent's sm-state)
+- **Monitor push loop** — when `monitor_push_enabled: true`, posts a
+  flat telemetry payload (cpu/mem/disk/swap/load + per-agent state)
+  to `<backendUrl>/monitor/server/heartbeat` every
+  `monitor_push_interval_seconds`. This replaces the standalone
+  `harborforge-monitor` daemon — the plugin's lifecycle (gateway
+  start/stop) bounds the loop, so a separate supervisor isn't needed.
+  Use the same `apiKey` value the standalone monitor's
+  `HF_MONITER_API_KEY` carried.
+- **Monitor bridge** (optional) — HTTP server on
+  `127.0.0.1:<monitor_port>` that responds to `/telemetry` with a
+  Snapshot. Useful when the standalone monitor is still present and
+  you want it to enrich its push payload from the plugin's view of
+  agents. Disable by setting `monitor_port: 0`.
 - **Calendar scheduler** — heartbeats `<backendUrl>/calendar/agent/
   heartbeat` every interval, receives any TimeSlots due to fire, and
   dispatches them through `HostAPI.WakeAgent` (state-aware queue
@@ -64,15 +74,24 @@ And configure at `~/.plexum/plugins/harbor-forge/config.json`:
 
 ```json
 {
-  "backendUrl": "https://monitor.hangman-lab.top",
+  "backendUrl": "https://hf-api.hangman-lab.top",
   "identifier": "server-t3",
   "apiKey": "g1_xxx",
-  "monitor_port": 9100,
+  "monitor_push_enabled": true,
+  "monitor_push_interval_seconds": 30,
+  "monitor_port": 0,
   "calendar_enabled": true,
   "calendar_heartbeat_interval_seconds": 30
 }
 ```
 
+Replacing the standalone `harborforge-monitor` container: lift the
+container's `HF_MONITER_API_KEY` into `apiKey`, set
+`monitor_push_enabled: true`, then `docker rm -f harborforge-monitor`
+once you've confirmed the plugin's pushes are landing (the backend's
+`server_states.last_seen_at` should keep advancing without the
+container running).
+
 Restart the host (`systemctl --user restart plexum`) and verify:
 
 ```bash