add deinstall steps into the readme

[keltik85]

i want to wipe it from anything it touched, memory, settings, claude.md etc.

[zhangaman]

2nd this request. I followed the instructions in README.md Steps 1 & 2 to install but now do not know how to uninstall it. I have not cloned the repo to my local so do not have a local access to the script (./bin/gstack-uninstall).