Clarify recommended install location for multi-host setups

Hi, I had a question about the intended install pattern for GStack.

The README currently shows Claude users installing to `~/.claude/skills/gstack`, while other agents use something like `~/gstack`. I’m using multiple hosts, and I’d prefer to keep tools in one workspace folder, for example:
`
sh
~/Workspace/tools/gstack
`
From looking at setup, it seems like GStack can already be cloned anywhere and then registered into each host with:
`
./setup --host codex
`
`
./setup --host claude
`
So the mental model would be:

- one canonical GStack checkout wherever the user keeps tools
- host skill directories are registrations/symlinks/copies from that checkout
- avoid duplicate real clones in different host folders

Is that the intended pattern?

I saw related discussion in #687, #1161, and the larger host/runtime RFC in #289 / #364. I’m not trying to reopen the larger runtime-contract work here. I’m only asking whether a small README clarification would be useful.

If yes, I’d be happy to open a tiny docs PR adding a short “Choose where GStack lives” section to the README.

Start with the README install instructions and the setup --host codex and setup --host claude commands described in the issue. Check the related discussions in #687, #1161, #289, and #364, then add a short README section documenting the confirmed multi-host install pattern and canonical checkout location.

Written by the indexing model from the issue text.