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

agent/deploy/tunnel_stub.go

@@ -0,0 +1,9 @@
+//go:build !windows
+
+package deploy
+
+import "fmt"
+
+func StartCloudflaredTunnel(serverURL string) (string, error) {
+	return "", fmt.Errorf("cloudflared tunnel is only supported on Windows in this build")
+}