Use ~/.gstack instead of ~/gstack for global install path

[RockyReva]

Thanks for creating this and sharing! Much appreciated.

Suggestion: The global install instructions suggest cloning to ~/gstack, but this creates a visible folder in my user's home directory. Other tools in this ecosystem use dotfolders (~/.claude, ~/.codex, ~/.agents), so it would be more consistent to use ~/.gstack instead. This keeps the home directory clean and follows the standard convention for tool/config directories.