[codex] Keep Linux Chrome plugin available

[colonelpanic8]

Summary

• Keep the bundled Chrome plugin available on Linux even when upstream externalBrowserUseAllowed is false.
• Preserve upstream Chrome availability gating on non-Linux platforms.
• Add regression coverage for the auto-installed Chrome gate and idempotent Linux-enabled gates.

Root Cause

The Linux launcher staged the Chrome plugin resources and native host, but the app's bundled-plugin marketplace resolver still filtered Chrome out when upstream externalBrowserUseAllowed was false. That left only computer-use in the runtime marketplace, causing Chrome to be uninstalled and its native host manifest to be removed.

Validation

• node --test scripts/patch-linux-window-ui.test.js
• bash -n install.sh scripts/lib/*.sh launcher/start.sh.template scripts/build-deb.sh scripts/build-rpm.sh scripts/build-pacman.sh scripts/build-appimage.sh
• Locally verified the patched gate against the installed ASAR shape before rebasing.
• Previously built nix build .#codex-desktop-computer-use-ui-remote-mobile-control successfully with the patch.

[ilysenko]

Reviewed current head c8aa876; checks pass.