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/components/systems-table/systems-table-columns.tsx

@@ -38,7 +38,7 @@ import {
 	secondsToUptimeString,
 } from "@/lib/utils"
 import { batteryStateTranslations } from "@/lib/i18n"
-import { connectedWiFi, strongestWiFiSignal } from "@/lib/wifi"
+import { connectedWiFi, strongestWiFi, strongestWiFiSignal } from "@/lib/wifi"
 import type { SystemRecord } from "@/types"
 import { SystemDialog } from "../add-system"
 import AlertButton from "../alerts/alert-button"
@@ -358,12 +358,10 @@ export function SystemsTableColumns(viewMode: "table" | "grid"): ColumnDef<Syste
 			sortUndefined: "last",
 			cell(info) {
 				const connections = connectedWiFi(info.row.original)
-				if (!connections.length) {
+				const strongest = strongestWiFi(connections)
+				if (!strongest) {
 					return null
 				}
-				const strongest = connections.reduce((best, current) =>
-					(current[1].r ?? Number.NEGATIVE_INFINITY) > (best[1].r ?? Number.NEGATIVE_INFINITY) ? current : best
-				)
 				const displayedConnections = viewMode === "table" ? [strongest] : connections
 				const title = connections
 					.map(([id, wifi]) => `${id}${wifi.s ? ` (${wifi.s})` : ""}: ${wifi.r === undefined ? "—" : `${wifi.r} dBm`}`)