fix: centralize gstack skill installs

[ypaymoore-lab]

Summary

• centralize gstack installs around a single canonical source repo
• point Claude and Codex-compatible discovery layers back to the same generated skills
• update docs and setup tests for the new shared install model

Verification

• bun test test/gen-skill-docs.test.ts -t 'setup script validation'
• bun test test/gen-skill-docs.test.ts -t 'Codex preamble resolves runtime assets from repo-local or global gstack roots'

[time-attack]

@16francej — recommend closing this PR because #1819 now owns the one-checkout direction, while this change lacks migration validation and has unresolved safety problems in its linking approach. Credit this PR’s one-checkout design intent in #1819, then close it as superseded.

[capy-ai]

Thanks for this, @ypaymoore-lab. The fix is now on main: the install model was reworked (install registry, ./setup --status, per-host staged installs) (see v1.91.16.0, #3014). Closing as superseded; if you still see the problem on the latest gstack, please open an issue with ./setup --status output.