fix(config): read POSIX fallback on Windows

[ousamabenyounes]

Fixes #723

Summary

• Add a Windows-only fallback from %APPDATA%\caveman\config.json to ~/.config/caveman/config.json when XDG_CONFIG_HOME is unset.

• Cover the silent fallback case with a simulated win32 config-resolution regression test.

Test verification (RED / GREEN)

RED before production fix:

node tests/test_repo_local_config.js
'full' !== 'lite'

GREEN after fix:

node tests/test_repo_local_config.js
13 passed, 0 failed

node tests/test_symlink_flag.js
12 passed, 0 failed

npm test
# tests 112
# pass 112
# fail 0