Codex generated skills still carried Claude defaults

@garrytan heads up: I opened PR #1162 for this Codex-host issue: https://github.com/garrytan/gstack/pull/1162

Problem: the Codex generated skill output under `.agents/skills/gstack-*` existed, but it was still carrying Claude-host behavior in important places:

- `MODEL_OVERLAY: claude` showed up in Codex-generated skills.
- The generated body injected the Claude model-specific behavioral patch instead of a GPT/Codex patch.
- Codex-host skills still contained Claude-only phrasing like `Agent tool`, `Claude subagent`, and `subagent_type: "general-purpose"`.
- Small no-preamble skills like `/freeze`, `/careful`, `/guard`, `/unfreeze`, and `/gstack-upgrade` missed host/model adaptation entirely.

PR #1162 fixes that by:

- defaulting Codex-host generation to `gpt-5.4` unless `--model` is explicit;
- adding a Codex host runtime patch to every generated Codex skill;
- rewriting Claude-only tool/subagent vocabulary for Codex output;
- making spawned skill-review agents load the full named `SKILL.md`, not a tiny prompt excerpt;
- adding regression tests that iterate every generated Codex skill and assert the Codex/GPT patches are present and Claude defaults are absent.

Verification on the PR branch:

- `bun run gen:skill-docs --host all`
- `bun run skill:check`
- `bun test test/gen-skill-docs.test.ts`
- `bun test test/host-config.test.ts test/gen-skill-docs.test.ts`
- `bun test`

PR: https://github.com/garrytan/gstack/pull/1162

Start with the generator and host configuration behind .agents/skills/gstack-* and inspect PR #1162. Run bun run gen:skill-docs --host all, bun run skill:check, and the named gen-skill-docs and host-config tests; done means generated Codex skills contain the Codex/GPT adaptations and no Claude defaults.

Written by the indexing model from the issue text.