[FR] write config files into XDG_CONFIG_HOME

[laur89]

As-is:
Config files are written into user homedir @ ~/.plandex-home-v2/

To-be:
Follow the xdg spec and write them into $XDG_CONFIG_HOME instead (~/.config/ in most cases).

If config will be versioned, then I suppose $XDG_CONFIG_HOME/plandex/v2/ would make sense.

As discussed in discord