Enable SQL server agent service by T-SQL script or PowerShell
It will depend on what version of PowerShell you are running, but this command can be used from at least 3.0 and up:
Set-Service SQLSERVERAGENT -StartupType Disabled


The alternative would be to use WMI, or you can use SQL Server WMI (microsoft.sqlserver.management.smo.wmi namespace).
In T-SQL there is no mechanism native that lets you interact with the services. Outside of just using xp_cmdshell and calling PowerShell or cmd commands.