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/system.go

@@ -165,7 +165,6 @@ const (
 
 // Core system data that is needed in All Systems table
 type Info struct {
-	// Always serialize the current snapshot, including null on unsupported agents.
 	Hostname      string `json:"h,omitempty" cbor:"0,keyasint,omitempty"` // deprecated - moved to Details struct
 	KernelVersion string `json:"k,omitempty" cbor:"1,keyasint,omitempty"` // deprecated - moved to Details struct
 	Cores         int    `json:"c,omitzero" cbor:"2,keyasint,omitzero"`   // deprecated - moved to Details struct
@@ -194,7 +193,7 @@ type Info struct {
 	Battery        Battery            `json:"bat,omitzero" cbor:"23,keyasint,omitzero"`   // [percent, charge state]
 	RootDiskName   string             `json:"rdn,omitempty" cbor:"24,keyasint,omitempty"` // custom name for root disk (set via FILESYSTEM=device__name)
 	PackageUpdates []uint16           `json:"pu,omitempty" cbor:"25,keyasint,omitempty"`  // [totalUpdates, securityUpdates] (security omitted if unknown)
-	WiFi           map[string]WiFi    `json:"wifi" cbor:"26,keyasint"`
+	WiFi           map[string]WiFi    `json:"wf,omitempty" cbor:"26,keyasint,omitempty"`  // connected Wi-Fi interfaces
 }
 
 // Data that does not change during process lifetime and is not needed in All Systems table