wifi: omit empty info snapshot and tidy up

- Info.WiFi uses json "wf" with omitempty so systems without Wi-Fi no
  longer store/broadcast "wifi":null
- move osascript exec into wifi_darwin.go; drop unused commandRunner
- share strongest-connection logic between table cell and sorting
- trim agent/wifi README

internal/entities/system/wifi_test.go

@@ -33,8 +33,8 @@ func TestWiFiWireSnapshot(t *testing.T) {
 		if err = json.Unmarshal(encoded, &info); err != nil {
 			t.Fatal(err)
 		}
-		if _, ok := info["wifi"]; !ok {
-			t.Fatal("current absence must be explicit")
+		if _, ok := info["wf"]; ok != (len(wifi) > 0) {
+			t.Fatalf("wf present = %v for snapshot %v", ok, wifi)
 		}
 	}
 }