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

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.

Locate the global installation instructions and all references to the `~/gstack` path. Update the documented path to `~/.gstack`, then check the surrounding instructions for consistency and verify that no old global-install path remains.

Written by the indexing model from the issue text.