Add tool to await existing background terminal(s)

[Tyriar]

Currently when an agent launches a background terminal like a watch task it can only check the output. This results in it doing interesting workarounds like trying to run echo "finished" or sleep n to wait for it. A tool to await specific terminal(s) to either complete (in the case of a long running compile) or wait for some specific timeout would help prevent these workarounds from being used.