callback hook for launchers

It would be great, when possible, to use a callback hook in launchers to signal when a launcher is ready. This could be used to return connection info from the controller to a Cluster object without needing access to the filesystem.

Unfortunately, we can't in general assume the controller or engines *can* call back to the host requesting the launch. So we need to keep the 'watch for files and copy them when they show up' behavior available, and perhaps as the default. However, for the simple Local launcher case, network callbacks should work nicer.

Start by tracing how launchers communicate with the controller and Cluster object, especially the Local launcher and the existing filesystem-watch behavior. Define how a readiness callback would return connection information while preserving the filesystem fallback, then verify both paths for launcher startup.

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