Add companion application

cmd/companion/open_windows.go

@@ -0,0 +1,15 @@
+//go:build windows
+
+package main
+
+import "os/exec"
+
+// openOSPath asks Windows to open a file in the default app, or a folder in
+// Explorer. `cmd /c start` handles default-handler dispatch for files; the
+// empty "" argument is the window title required by start.
+func openOSPath(p string, isFolder bool) error {
+	if isFolder {
+		return exec.Command("explorer.exe", p).Start()
+	}
+	return exec.Command("cmd", "/c", "start", "", p).Start()
+}