Replace user-facing 'Agent' references with 'Runner'

[juliobbv]

There were a few "user-facing" references to Agent, namely in both the run and config scripts, so I renamed them to Runner for consistency.

I also simplified the 'file unblocking' in config.cmd and run.cmd to remove the no-longer-needed Agent files while I'm here.

I decided to leave the code references (e.g. variable names) as Agent, as some of the server types include Agent in there. Replacing with runner would mean looking at code like this:

var taskRunnerSession = new TaskAgentSession(sessionName, runner, systemCapabilities);

...which I find more awkward than just leaving them as agent.

[bryanmacfarlane]

Thanks for taking the time to write a good description for this PR!