Explain how to configure a missing Codex executable · #11345

The spawn error now names the configured executable and points to its Binary path setting on the server. For the default codex command, it explains why a desktop installation may not put codex on PATH. This addresses the setup-guidance option in #10514; app-bundle discovery remains separate.

Eleven focused provider checks pass, including a real missing-executable probe through the registry. Four expectations fail against the parent. Server typecheck and scoped lint pass. Native Electron displays simulated before/after provider status; the updated message fits the 1000px client without truncation. No generation turn or native macOS/Windows check was run. CI and both Macroscope checks pass, with approval and no inline findings.

No merge. Internal Fable refinement remains pending. Stacked on #11298 with its existing holds. GitHub image upload remains pending because the background browser connection is unavailable. The server message is shared across clients; no executable selection or provider defaults change.

Before

After