Codex design workflows report DESIGN_NOT_AVAILABLE due to broken fallback paths + missing design install surfaces

Summary

Codex-hosted design skills can incorrectly report DESIGN_NOT_AVAILABLE even when the design binary exists. The failure comes from a combination of broken fallback path generation and missing design install surfaces in the Codex runtime.

What breaks

There are three linked problems:

1. Generated fallback paths in Codex setup blocks prepend $HOME to env-var-backed paths like $GSTACK_DESIGN and $GSTACK_BROWSE, producing invalid doubled-home paths.
2. The Codex global runtime root does not expose design/dist, so even a correct fallback path has no global design binary to execute.
3. The repo-local .agents/skills/gstack sidecar also omits design, so the local-first resolution path fails in repos that have a sidecar install.

Reproduction

Environment:

• gstack current main
• Codex host install

Observed from generated Codex design skill setup:

• fallback for D becomes $HOME$GSTACK_DESIGN/design
• fallback for B becomes $HOME$GSTACK_BROWSE/browse

In Codex, those env vars already resolve to absolute paths, so the resulting lookup becomes effectively:

• /home/user/home/user/.codex/skills/gstack/design/dist/design
• /home/user/home/user/.codex/skills/gstack/browse/dist/browse

That causes DESIGN_NOT_AVAILABLE / browse-unavailable behavior even when the underlying binary exists elsewhere.

Root cause

Verified against current source:

• scripts/resolvers/design.ts
  • generateDesignSetup() builds fallback with $HOME${ctx.paths.designDir...} and $HOME${ctx.paths.browseDir...}
  • generateDesignMockup() builds fallback with $HOME${ctx.paths.designDir...}
• scripts/resolvers/browse.ts
  • generateBrowseSetup() builds fallback with $HOME${ctx.paths.browseDir...}
• hosts/codex.ts
  • Codex runtime assets include bin, browse/dist, browse/bin, gstack-upgrade, ETHOS.md, but not design/dist
• setup
  • create_codex_runtime_root() does not link design/dist
  • create_agents_sidecar() does not link design

So this is not just a generated text bug. It is generation drift plus install-surface drift.

Expected behavior

For Codex installs:

• generated setup blocks should resolve valid fallback paths for both design and browse binaries
• ~/.codex/skills/gstack/design/dist/design should exist after setup when design workflows are advertised
• repo-local .agents/skills/gstack/design/dist/design should exist when the sidecar is present
• design-shotgun preflight should return DESIGN_READY when the binary is installed

Proposed fix

1. Fix shared resolver generation for env-var-backed host paths

   • update scripts/resolvers/design.ts
   • update scripts/resolvers/browse.ts
   • do not prepend $HOME when the host path is already $GSTACK_*

2. Add design/dist to Codex runtime assets

   • update hosts/codex.ts

3. Patch the actual install helpers in setup

   • create_codex_runtime_root() should link design/dist
   • create_agents_sidecar() should link design

4. Add regression coverage for:

   • Codex host asset manifest includes design/dist
   • generated Codex design setup has valid D and B fallbacks
   • generated Codex browse setup does not emit $HOME$GSTACK_BROWSE
   • setup helper coverage for both global runtime root and repo-local sidecar

Why this matters

Right now Codex design workflows degrade into fallback not because the feature is optional, but because the generated/install paths are broken. That makes the host integration look unreliable even when the underlying binary exists.

I can send a PR for this from a fork if the maintainers want the fix bundled with regression tests.