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/site/src/lib/wifi.ts

@@ -3,14 +3,22 @@ import type { SystemRecord, WiFi } from "@/types"
 // Current system info is independent of the selected historical chart window.
 // No fallback to history: missing data, disconnect and offline all hide the panel.
 export function connectedWiFi(system: Pick<SystemRecord, "status" | "info">): [string, WiFi][] {
-	return system.status === "up" ? Object.entries(system.info?.wifi ?? {}).sort(([a], [b]) => a.localeCompare(b)) : []
+	return system.status === "up" ? Object.entries(system.info?.wf ?? {}).sort(([a], [b]) => a.localeCompare(b)) : []
+}
+
+/** Strongest connection by RSSI, falling back to the first when none report a signal. */
+export function strongestWiFi(connections: [string, WiFi][]): [string, WiFi] | undefined {
+	let strongest = connections[0]
+	for (const connection of connections) {
+		if ((connection[1].r ?? -Infinity) > (strongest[1].r ?? -Infinity)) {
+			strongest = connection
+		}
+	}
+	return strongest
 }
 
 export function strongestWiFiSignal(system: Pick<SystemRecord, "status" | "info">): number | undefined {
-	const signals = connectedWiFi(system)
-		.map(([, wifi]) => wifi.r)
-		.filter((signal): signal is number => signal !== undefined && Number.isFinite(signal))
-	return signals.length ? Math.max(...signals) : undefined
+	return strongestWiFi(connectedWiFi(system))?.[1].r
 }
 
 export function wifiColor(id: string): string {