A literal XDG_CONFIG_HOME='~/.config' passed validation and let Goose
write its config under ./~ in magpie's working directory. The same
problem affected other folder variables consumed without expansion.
Only retain home-relative paths for the Pi, Hanako and T3 variables
whose readers expand them. Ignore other unrooted values and report them
through CheckEnv, preserving the environment passed to child processes.
Add regression coverage for variable validation and an actual Goose
config write, while keeping existing supported tilde paths working.
Zhizhen He
50686784bc