Name and order the agent and config code

The names that only the agent or only the config uses start with agent
or config: the instructions file of the agent and the endpoint values of
config.json. Config and ModelConfig stay generic, because the daemon,
the probe, the model client, and the compactor read them.

The data directory is in datadir.go, with its tests in datadir_test.go,
because the login, the terminal client, the daemon, and the probe use
it, and it is not a part of config.json. The declarations of these files
follow the order of the go-code skill. The behavior does not change.

Issue: 947bddb