tools(agent-loop): make install-grok.ps1 parse on Windows PowerShell 5.1

$TaskName: in a double-quoted string is read as a drive-qualified variable, so the script died before -RunOnce.

tools/agent-loop/install-grok.ps1

@@ -48,5 +48,5 @@ $settings = New-ScheduledTaskSettingsSet -MultipleInstances IgnoreNew -StartWhen
              -ExecutionTimeLimit (New-TimeSpan -Hours 2) -AllowStartIfOnBatteries -DontStopIfGoingOnBatteries
 Register-ScheduledTask -TaskName $TaskName -Action $action -Trigger $trigger -Settings $settings `
   -Description 'Jyotisha agent loop: Grok executes agent/todo issues' -Force | Out-Null
-Write-Host "registered $TaskName: every $($cfg.tick_minutes) min while you are logged on"
+Write-Host "registered ${TaskName}: every $($cfg.tick_minutes) min while you are logged on"
 Write-Host "pause: create $HOME\.config\jyotisha-agent\STOP   log: $HOME\.jyotisha-agent\state\grok-tick.log"