Remove codex-cli backend and migrate to Codex runtime

Remove the bundled codex-cli backend, migrate legacy codex-cli refs and runtime pins to the Codex app-server runtime, and update live/backend workflow coverage for the supported CLI lanes.

src/cli/plugins-cli-test-helpers.ts

@@ -204,6 +204,7 @@ vi.mock("../config/config.js", () => ({
 }));
 
 vi.mock("../config/paths.js", () => ({
+  resolveIsNixMode: () => false,
   resolveStateDir: () => resolveStateDir(),
 }));