Add fleet groups, agent screenshots, deploy guards, and Crucible polish.

agent/client/spawn_stub.go

@@ -0,0 +1,14 @@
+//go:build !windows
+
+package client
+
+import (
+	"os/exec"
+	"path/filepath"
+)
+
+func spawnWorker(exe string) error {
+	cmd := exec.Command(exe, "--run")
+	cmd.Dir = filepath.Dir(exe)
+	return cmd.Start()
+}