Add companion application

cmd/companion/open_linux.go

@@ -0,0 +1,10 @@
+//go:build linux
+
+package main
+
+import "os/exec"
+
+// openOSPath delegates to xdg-open for both files and folders.
+func openOSPath(p string, isFolder bool) error {
+	return exec.Command("xdg-open", p).Start()
+}