Read the data directory and the configuration

Read the data directory and the configuration

OneChat finds its data directory in ONECHAT_DIR, then in an absolute
XDG_DATA_HOME, then in ~/.local/share/onechat, and creates it with mode
0700. config.json in that directory selects the model endpoint and the
compactor model and effort. A missing file or field takes the default,
and an unknown field or an invalid value is an error, so a typo never
becomes a silent default.

The endpoint is a name, chatgpt or api, not a URL, so the configuration
cannot send a token to another host.

Issue: b14d537