fix: add codex skill metadata for gstack discovery

[malikrohail]

Summary

• generate agents/openai.yaml metadata for every generated Codex skill
• add a root agents/openai.yaml bundle metadata file for gstack
• copy Codex metadata in the Codex session-runner helper so tests mirror real installs
• keep the change scoped to Codex skill discovery metadata only

Problem

On this machine, the Codex $ picker only surfaced gstack-upgrade reliably even though all gstack-* skill symlinks existed under ~/.codex/skills/.

The missing piece was Codex metadata:

• the root gstack bundle had no agents/openai.yaml
• the generated .agents/skills/gstack-* Codex skills also had no agents/openai.yaml

Fix

• write agents/openai.yaml for each generated Codex skill using the gstack-* display name
• add root bundle metadata at agents/openai.yaml so gstack itself is exposed as a bundle
• update the Codex temp-home installer helper to copy agents/openai.yaml when present

Related to #333.

Verification

• bun run gen:skill-docs --host codex
• bun test test/gen-skill-docs.test.ts test/codex-e2e.test.ts
• bun test
• ./setup --host codex
• real codex exec probe after setup:
  • before this change, the visible gstack-* set was incomplete in the picker symptom reported by the user
  • after this change, codex exec lists:
    gstack, gstack-autoplan, gstack-benchmark, gstack-browse, gstack-canary, gstack-careful, gstack-cso, gstack-design-consultation, gstack-design-review, gstack-document-release, gstack-freeze, gstack-guard, gstack-investigate, gstack-land-and-deploy, gstack-office-hours, gstack-plan-ceo-review, gstack-plan-design-review, gstack-plan-eng-review, gstack-qa, gstack-qa-only, gstack-retro, gstack-review, gstack-setup-browser-cookies, gstack-setup-deploy, gstack-ship, gstack-unfreeze, gstack-upgrade

[malikrohail]

@garrytan verified on March 22, 2026.

This PR is intentionally limited to Codex discovery metadata:

• generate agents/openai.yaml for each generated Codex skill
• add a root gstack bundle metadata file
• keep setup/layout unchanged
• keep the fix measurable with a real ./setup --host codex + codex exec run

On my machine, this was the difference between seeing only a partial gstack result in the Codex picker symptom and seeing the full gstack-* skill set after setup.