Replace large byte-over-IPC desktop save flow with staged filesystem writes and native commit validation.
Remove legacy save commands, narrow frontend fs scope to exact staging files, and document the Codex desktop Node workaround for Vite/Vitest on macOS.
Verification: cargo test, cargo clippy -- -D warnings, pnpm run test:upstream, pnpm run test:studio, pnpm run build:studio, pnpm --filter hop-desktop tauri build --debug --bundles app.
