initial: HarborForge plugin for Plexum (port of OpenclawPlugin)

Plugin id `harbor-forge` mirrors the OpenClaw counterpart's runtime
surface on top of the Plexum SDK:

  * eager activation — Monitor bridge + Calendar scheduler boot at
    host start, before any agent turn fires
  * monitor bridge: HTTP 127.0.0.1:<monitor_port> serving /telemetry
    + /health for HarborForge.Monitor
  * calendar scheduler: heartbeats <backendUrl>/calendar/agent/
    heartbeat, dispatches returned slots via HostAPI.WakeAgent
    (state-aware queue, depth-1 replace-newest), tracks active slot
    state in-memory, terminal status pushed back to backend
  * 9 harborforge_* tools (status / telemetry / monitor_telemetry /
    calendar_{status,complete,abort,pause,resume} / restart_status)

Key differences from OpenClaw equivalent:
  * api.spawn → HostAPI.WakeAgent (new SDK primitive)
  * api.getAgentStatus → HostAPI.ReadAgentState (existing)
  * --install-monitor / --install-cli not included; Monitor + hf CLI
    deploy via the HangmanLab.Server.T3 docker compose layer

Initial drop. TODO before v1 ship:
  * tool ctx → calling-agent-id: SDK doesn't currently expose; v1
    falls back to a single-active-slot heuristic in
    main.bestEffortAgentID
  * tests for the bridge + scheduler

internal/telemetry/diskstat_linux.go

@@ -0,0 +1,20 @@
+package telemetry
+
+import "syscall"
+
+type diskStat struct {
+	blockSize uint64
+	blocks    uint64
+	bavail    uint64
+}
+
+func statfs(path string, out *diskStat) error {
+	var fs syscall.Statfs_t
+	if err := syscall.Statfs(path, &fs); err != nil {
+		return err
+	}
+	out.blockSize = uint64(fs.Bsize)
+	out.blocks = fs.Blocks
+	out.bavail = fs.Bavail
+	return nil
+}