Add appcast-driven Linux update flow

[avifenesh]

Summary

• Use the upstream Codex Sparkle appcast as the Linux updater source of truth, including the appcast enclosure URL and upstream version.
• Enable the existing app update UI on Linux and check for appcast updates on app launch.
• Add an opt-in Linux feature selection flow before installing a ready update, with the selection persisted under XDG config and replayed into rebuilds.
• Rebuild a ready package when the user changes optional features before install, and hide developer-only fixture features from native prompts.

Validation

• curl -fsSI https://persistent.oaistatic.com/codex-app-prod/appcast-x64.xml
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test -p codex-update-manager
• cargo build -p codex-update-manager
• node --test scripts/patch-linux-window-ui.test.js
• node --test linux-features/*/test.js
• git diff --check
• codex-update-manager features --json with isolated XDG config
• CODEX_BOOTSTRAP_NONINTERACTIVE=1 scripts/bootstrap-wizard.sh with isolated feature config
• Computer Use visual smoke: opened the native Codex Desktop update feature checklist and verified cancel returns structured JSON with cancelled: true.