Fix Codex design binary path resolution

[mamedov]

Summary

• fix Codex fallback path generation for design and browse when host paths are env-var-backed
• expose design/dist in the Codex runtime root and repo-local sidecar install
• add regression coverage for generated Codex setup blocks and install surfaces

Fixes #1159.

Root cause

Codex uses env-var-backed host paths like $GSTACK_DESIGN and $GSTACK_BROWSE. The generated fallback shell snippets were prepending $HOME unconditionally, which produced invalid doubled-home paths. At the same time, the Codex runtime setup was not linking design/dist, and the repo-local sidecar was not linking design at all.

Testing

• bun run gen:skill-docs --host all
• bun test test/codex-design-paths.test.ts test/host-config.test.ts test/gen-skill-docs.test.ts

[ohdelicescreative]

Problem still exists in 1.26.4.0

[time-attack]

@16francej — recommend closing this PR. This is superseded by #2056, which provides the broader shared resolver across hosts. Landing #1160 separately would duplicate that work while omitting other hosts. Please port and credit the Codex design runtime, sidecar, setup, and tests in #2056.

[capy-ai]

Thanks for tracking this down, @mamedov, and thanks @ohdelicescreative for confirming it. Main now links design/dist into the Codex runtime root (hosts/codex.ts, setup's _link_runtime_dists), and the doubled-$HOME fallback paths are gone since the resolver rewrite in v1.91.19.0. Closing as fixed on main. If /design-* on Codex still can't find the binary on current main, please reopen with the DESIGN_NOT_AVAILABLE line it prints.