Add host-mode heartbeat: vetting-agent host + last-seen badge

vetting-agent gains a `host` subcommand that runs as a systemd service
installed by the quick-register one-liner, POSTing every 30s to
/api/v1/hosts/{mac}/heartbeat so the dashboard tile shows "online" or
"Nm ago" without waiting on WoL. Ships dormant client code for the
Phase 2 reboot_for_vetting command so the server can flip it on later
without a binary redeploy.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

cmd/vetting/main.go

@@ -149,9 +149,10 @@ func main() {
 	}
 
 	router := httpserver.NewRouter(httpserver.Deps{
-		UI:      ui,
-		Agent:   agentAPI,
-		LiveDir: cfg.PXE.LiveDir,
+		UI:            ui,
+		Agent:         agentAPI,
+		LiveDir:       cfg.PXE.LiveDir,
+		AgentAssetDir: cfg.Agent.AssetDir,
 	})
 
 	srv := &http.Server{